Carrier Aggregation (CA) is a technique used in LTE-Advanced to increase the peak data rate (i.e., maximum available speed) of a 4G LTE network. By aggregating multiple channels together a mobile network operator can increase the total available bandwidth of a single transmission, and thereby increase the bitrate and capacity of the network.
Before LTE-A, a mobile network operator could only use additional LTE bands to increase capacity (i.e., total number of users), by distributing traffic across multiple bands. While this is an effective way to provide more airtime to any one particular user and thereby improving available data rates during busy periods, it does not increase the peak data rate. Carrier Aggregation was introduced in 3GPP Rel. 10 (2011).
With radio spectrum a rare commodity, CA has also helped to utilise smaller spectrum allotments which only support smaller channel widths and where traditional LTE would lead to very low peak data rates.
Carrier aggregation is supported in both FDD and TDD duplex modes, with mixed duplex FDD+TDD aggregation possible. Special Supplemental Downlink (SDL) and Supplemental Uplink (SUL) bands have also been introduced to improve peak data rates in one direction only, often to utilise isolated pockets of radio spectrum unsuitable for duplex.