Carrier Aggregation is a technology to combine two or more carriers into one data channel to enhance the data capacity. It is possible to combine carriers in the same or different frequency bands. This is explained in the following figure.
Carrier aggregation
The easiest way to aggregate carriers is to take two or more contiguous channels within the same frequency band (intra-band, contiguous). It is also possible to combine two or more non-contiguous channels within the same band (intra-band, non-contiguous). The most challenging form is to aggregate two or more carriers from different frequency bands (intra-band; non-contiguous).
Carrier aggregation technology is implemented in most modern mobile communication systems, like LTE and WiMax. In IEEE 802.11 (Wi-Fi) it is possible to combine two contiguous channels into one big data channels. In IEEE 802.11 terminology this is called channel bonding.
The MAC entity is responsible for distributing data from each flow across the component carriers, a decision that is part of the implementation-specific scheduling approach in the downlink. Each component carrier has its own hybrid-ARQ entity, implying that hybrid-ARQ retransmissions must occur on the same component carrier as the original transmission. There is no possibility to move hybrid-ARQ retransmissions between carriers.
Types of Carrier Aggregation (CA)
Intra-band Carrier Aggregation: This form of carrier aggregation uses a single band. It is further divided up in to two parts:
Contiguous: This is the easiest form of LTE carrier aggregation to implement. In this the carriers are adjacent to each other. In this case you only need a single transceiver as the signal is considered as a single enlarged signal.
Non-contiguous: This one is slightly complicated as the carriers use the same operating band but are not adjacent to each other. So you need two transceivers, because the signal can’t be treated as a single signal, adding to complexity and cost.
This technology can be applied to either the FDD or TDD variants of LTE with a maximum of five component carriers, each with a bandwidth of up to 20 MHz, resulting in a total transmission bandwidth of up to 100 MHz
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