LTE Advanced Carrier Aggregation, CA, is one of the key techniques used to enable the very high data rates of 4G to be achieved.
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By combining more than one carrier together, it is possible to increase the bandwidth available and in this way increase the capacity of the link.
LTE carrier aggregation, CA, enables the most to be made of the available radio spectrum. Often bands are small as the availability of spectrum is limited in the sectors required and as a result, many country administrations have released as much as they can but this may only be as small bands. Carrier aggregation seeks to be able to utilize this and large bands equally effectively.
Carrier aggregation is supported by both formats of LTE, namely the FDD and TDD variants. This ensures that both FDD LTE and TDD LTE are able to meet the high data throughput requirements placed upon them.
It is necessary to be able to indicate to which component carrier in any aggregation scheme a grant relates. To facilitate this, component carriers are numbered.
When carriers are aggregated, each carrier is referred to as a component carrier. There are two categories:
- Primary component carrier: This is the main carrier in any group. There will be a primary downlink carrier and an associated uplink primary component carrier.
- Secondary component carrier: There may be one or more secondary component carriers.
There is no definition of which carrier should be used as a primary component carrier - different terminals may use different carriers. The configuration of the primary component carrier is terminal-specific and will be determined according to the loading on the various carriers as well as other relevant parameters.
In addition to this, the association between the downlink primary carrier and the corresponding uplink primary component carrier is cell-specific. Again there are no definitions of how this must be organized. The information is signaled to the terminal of user equipment as part of the overall signaling between the terminal and the base station.
