Page 297 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 283

CA_3A‐7A‐38A: Aggregates up to 20 MHz in bands 3 and 7 in which frequency
division duplex is used (FDD‐LTE). Additionally up to 20 MHz is aggregated in band 38
in which time division duplex (TDD‐LTE) is used.
In practice, carrier aggregation‐capable devices typically support several dozen dif-
ferent combinations to accommodate the diverse spectrum use of network operators
around the world. As a consequence, CA combination lists in the UECapabilityInformation
message have become very long. To keep the lists and resulting message sizes as small
as possible, mobile devices typically no longer send all supported bands and CA combi-
nations but only those that are applicable in the region in which a mobile device is
currently located. This can for example be determined based on the frequency bands a
device detects during network selection. In Europe, for example, devices would not
announce support for US bands and CA combinations and vice versa.

4.11.2 CA Configuration, Activation and Deactivation
In practice, carrier aggregation is implemented as follows. When a device moves from
RRC idle to RRC connected state as described above, only a single carrier is used. If
the network has not stored the device’s capability description from a previous connec-
tion it sends a UECapabilityEnquiry message that the mobile device answers with a
UECapabilityInformation message. This message contains, among many other param-
eters, the UE category that describes the sustainable datarate the device supports, the
supported frequency bands and carrier aggregation combinations. Carrier aggregation
capabilities are given separately for the uplink and the downlink direction. Typically,
carrier aggregation is only used in the downlink direction today.
Whether and when a connection to a mobile device is augmented with additional
carriers is implementation specific. A straightforward implementation found in prac-
tice today is that the network instructs mobile devices to always camp on the carrier
with the highest frequency it can find while it is in RRC‐idle state, even if the received
signal quality is lower than the signal quality of a carrier in a lower frequency band.
When the device then connects to the eNode‐B on this higher frequency, the eNode‐B
assumes that the mobile device can also receive the carrier on the lower frequency
band or bands and immediately configures carrier aggregation during connection
setup. The carrier used for both uplink and downlink transmission is referred to as
the Primary Component Carrier (PCC) or Primary‐Cell (PCell). Additional compo-
nent carriers are referred to as Secondary Component Carriers (SCC) or Secondary‐
Cells (SCell). Adding SCells during the connection setup procedure can be done in
less than 100 milliseconds.
Some networks use a more complicated procedure to find out whether it is possible
and necessary to configure secondary component carriers. In some networks the con-
figuration of carrier aggregation is only attempted when the eNode‐B detects that larger
amounts of data need to be transferred. If it is not certain that the device can receive all
carriers that the eNode‐B would like to aggregate, for example, because the device does
not camp on the highest frequency band active at a cell site, the eNode‐B can instruct
the mobile device to perform interband measurements to determine whether and at
which signal strengths potential SCells can be received. If sufficient signal strengths of
potential SCells are reported back the eNode‐B then configures carrier aggregation.
The advantage of this more complicated procedure is that higher‐frequency carriers are