Page 295 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 281

In the United States, initial spectrum assignments for LTE were significantly more
fractured as compared to Europe. As a consequence, carrier aggregation was initially
used to combine two or more 10 MHz carriers in different frequency bands to reach the
same aggregate speed as a single 20 MHz LTE Release 8 carrier. As an example, some US
network operators initially combined their 10 MHz spectrum in the 700 MHz band
with an additional 10 MHz carrier in the 1700/2100 MHz band for a total aggregate
bandwidth of 20 MHz. In the meantime, the spectrum landscape has become some-
what less fractured due to the local regulator making more spectrum available, network
operators trading their spectrum holdings among themselves to increase the adjacent
spectrum that can be accumulated to a single carrier, and reuse of spectrum for LTE
that was previously assigned to GSM, CDMA and UMTS.
At the time of publication, carrier aggregation was still considered a high‐end fea-
ture and the majority of LTE‐capable devices in use did not yet support the feature.
It could be observed, however, that aggregating two carriers was about to move to
the mid‐range device sector while high‐end devices supported the aggregation of 3
to 4 carriers.
It is important to note that carrier aggregation serves two purposes. On the one hand
it increases the theoretical peak datarate per user. On the other hand, however, the ris-
ing number of users being connected simultaneously and the rising data volumes per
user decreases this benefit in practice. Therefore, from a network capacity point of view,
carrier aggregation is much more useful for dynamically scheduling the downlink traffic
of many simultaneous users. With carrier aggregation, devices can receive data via dif-
ferent parts of the spectrum with different propagation properties without reconfigura-
tion. Depending on the changing signal conditions of all active devices, the cell’s
scheduler can quickly change the part of the spectrum in which data is transmitted to a
device without requiring devices to continuously change between the different bands.

4.11.1 CA Types, Bandwidth Classes and Band Combinations

Four different types of carrier aggregation have been defined:
In practice, network operators often aggregate two or more carriers in different
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frequency bands. This is referred to interband carrier aggregation and is the most
common form of carrier aggregation in use today.
If a network operator has more than 20 MHz of contiguous spectrum available in one
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frequency band, intra‐band contiguous carrier aggregation is used, e.g. 20 + 10 MHz.
The network operator could also make a 15 + 15 MHz split but this would be to the
disadvantage of devices that do not support the aggregation of several carriers in
this band.
In some cases a network operator might have been assigned several chunks of spectrum
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in a single frequency band that are not contiguous. Combining these carriers is
referred to as intra‐band non‐contiguous carrier aggregation.
In some countries, such as, for example, Sweden, network operators have acquired
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FDD (Frequency Division Duplex) and TDD (Time Division Duplex) spectrum (i.e. uplink
and downlink transmissions are on the same channel) and use carrier aggregation in
the downlink direction to combine one or more FDD carriers in the downlink
direction and one or more TDD carriers. This is referred to as interband FDD/TDD
carrier aggregation. In the uplink direction one of the FDD carriers is used.