Dual-carrier supporting two carriers in the downlink was introduced in Rel. 8. Ever since its commercial launch in 2010, it has spread rapidly and was deployed in about 160 network worldwide, as of early 2014. Carrier aggregation has evolved strongly to bring across bands and uplink aggregation in Rel. 9, 3 and 4 carriers in Rel. 10, followed by non-contiguous bands, up to 8 carriers and aggregation across cells (called as MultiFlow) in Rel. 11. A special-case called supplemental down link (SDL), enabled by Rel. 9, aggregates unpaired spectrum with the downlink of paired spectrum.
Carrier aggregation not only increases the peak data rates, but also increases date rates for all users, proportional to number of carriers being aggraded, for example, doubled rates for two carriers and quadrupled for four carriers.