Page 298 - From GMS to LTE

284 From GSM to LTE-Advanced Pro and 5G

reserved for carrier aggregation‐capable devices. Devices that are not CA‐capable do
not camp on those frequency bands due to the network‐configured preference for lower
frequency bands. Also, requiring measurements and only configuring SCells when
radio conditions are favorable prevents a loss of throughput. However, there are disad-
vantages of this approach as well. When CA is configured, many devices change the
radio type indicator in the status bar, e.g. from ‘LTE’ to ‘LTE+’, which happens much less
often if CA is not automatically configured during connection establishment.
Furthermore, waiting for a certain data throughput and the result of measurements
before configuring CA means that data transfers start with a lower speed which then
suddenly increases after a few seconds.
Typically, SCell configuration includes the setup of signal quality measurements so
the eNode‐B can dynamically schedule downlink data traffic across the different com-
ponent carriers. Figure 4.25 shows how CA is configured in practice.
Once SCells are configured with RRC messaging the eNode‐B can activate them at
any time on the MAC layer using a MAC‐layer control element. The SCells then become
available for use eight subframes later, i.e. after 8 milliseconds, and remain active until
the expiry of the sCellDeactivationTimer, if configured, or until deactivated again by the
network with another MAC‐layer control element.

UE eNodeB

RRC Connection Request (Transfer from RRC-Idle to RRC-Connected)
RRC Connection Setup
RRC Connection Setup Complete

Security Mode Command (Activate Ciphering)
Security Mode Complete

UE Capability Enquiry (optional, only sent if eNode-B does not re-use values)
UE Capability Information (UE class, bands, CA combinations)

RRC Reconfiguration Command
(Bearer Setup, Measurement Configuration, Configure SCells)
RRC Reconfiguration Complete

SCell activation/deactivation on MAC-layer as required

Figure 4.25 CA configuration during RRC connection establishment.