The proliferation of mobile data services has increased the demand for higher user rates and network capacity. While LTE supports flexible carrier bandwidth up to 20MHz, many operators have fragmented spectrum assets across different bands, while other operators have a block of spectrum wider than 20MHz bandwidth. LTE-Advanced provides a solution to the need for increased bandwidth by allowing operators to aggregate up to 5 carriers together, creating a fatter data pipe with a total bandwidth of up to 100MHz. It also enables operators to leverage all spectrum assets by allowing the component carriers to be from the same band or from across different bands.
LTE Carrier Aggregation (CA) was defined in 3GPP Rel-10. A UE in CA mode has one Primary Component Carrier (PCC) and one or more Secondary Component Carriers (SCC). PCC acts as the anchor carrier and can optionally cross-schedule data transmission on SCC. Rel-10 supports FDD+FDD and TDD+TDD aggregation, while FDD+TDD aggregation is currently being studied in 3GPP Rel-12. Unpaired spectrum can also be aggregated as Supplemental Downlink (SDL) to improve downlink capacity and user experience.
By increasing the aggregated bandwidth, CA improves user data rates proportionally. For example, by aggregating two 10MHz LTE carriers together, the peak, median and cell edge user rates can all be doubled. Additional benefits of using CA include, efficient resource utilization with dynamic load balancing across carriers. For bursty traffic, CA provides capacity boost beyond linear gain due to trunking efficiency. Alternatively, CA delivers much higher user experience given the same number of users and bursty traffic load.