Until 3GPP Release 10, we could achieve maximum 150 Mbps downlink throughput with 20 MHz bandwidth, but in 3GPP Release 10 a new mark of LTE was introduced where several frequencies can be aggregated. The new mark of LTE was called as LTE-Advanced and the new introduced feature in this release was named as Carrier Aggregation or with the abbreviated version CA. Carrier Aggregation allows to aggregate contiguous or non-contiguous carriers to widen bandwidth up to 100 MHz.
With simple explanation, let’s assume we have a carrier from Band7 (LTE 2600) with 10 MHz and another 20 MHz bandwidth from Band3 (LTE 1800). Now we combine these carriers and get a 30 MHz carrier.
Carrier Aggregation can be used for both FDD and TDD separately, as well as the combination of both technology.