CA is carrier aggregation which is a concept used in LTE-Advanced in order to increase the bandwidth of the systems. By increasing the bandwidth of the systems, it increases the bit rate. Carrier aggregation is a concept that is used in both TDD (Time division duplex) and FDD (Frequency division duplex) systems. In Carrier aggregation, each aggregated carrier is referred to as a CC (Component carrier). The component carrier can have a bandwidth of 1.4,3,5,10,15 or 20 MHz and a total of five component carriers can be aggregated because of which the maximum aggregated bandwidth of 100 MHz can be achieved. The easiest way to arrange aggregation would be to use contiguous component carriers within the same operating frequency band which is so-called intra-band contiguous. The easiest way to arrange aggregation would be to use contiguous component carriers within the same operating frequency band, so-called intra-band contiguous. For non-contiguous allocation, it could either be intra-band i.e all the component carriers belong to the same frequency band and another inter-band i.e all the component carriers can belong to the different frequency bands.
Three CA configurations that are defined are intra-band contiguous FDD, Intra-band contiguous TDD, and Inter-band FDD. When carrier aggregation is used there are a number of serving cells, one for each component carrier. The coverage of the serving cells may differ e.g due to that CCs on different frequency bands will experience different path loss. Different Component carriers are planned to provide different coverage i.e different cell sizes.