As the demand for wireless communications is growing rapidly, the network traffic is increasing in a high speed. The future network deployment will be denser, and also consider both the evolvements of macro base stations and small base stations. The wide use of small cells and the corresponding demand for non-ideal backhaul spawned Dual Connectivity technology, namely a kind of carrier aggregation that a terminal connects to a macro base station and a small cellular base station simultaneously.
Dual Connectivity is essentially carrier aggregation under non-ideal Backhaul with different carrier aggregation sites. The typical scenario is that one site is a macrocell base station, another is a small cell base station. Macrocell and small cell is connected via a standard X2 interface. For R10, carriers in the carrier aggregation are all scheduled by one scheduler; while for Dual Connectivity, macrocells and small cell scheduler manages radio resources respectively, which makes base stations necessary to coordinate with each other. This illustrates the essential distinction between Dual Connectivity and carrier aggregation from the perspective of the system architecture. Mobility management is controlled by the macro base station in Dual Connectivity, while small base stations provide additional user plane capacity, to increase user data throughput.
One relatively big advantage of Dual Connectivity technology is the relatively low requirements for backhaul link between the macrocell and small cell compared with carrier aggregation, including allowing macrocells and small cellular systems are not synchronized. Also, Dual Connectivity optimizes system capacity for macro base stations and small base stations, and solves the problem caused by users at the cell edge. However, the disadvantage is that due to non-ideal inverse backhaul might bring some delay, and this technology requires both the user and the base station make improvements to the corresponding evolution.
In addition, for the same site of LTE and WLAN systems, perhaps the direct carrier aggregation is a more appropriate approach; and for different sites of LTE and WLAN systems, probably the Dual Connectivity is a better choice. In fact, R12 has introduced LTE and WLAN systems to interoperate in a wireless access level in 3GPP. Currently, some have proposed Dual Connectivity between the LTE system and the LAA (Licensed Assisted Access) system. All in all, Dual Connectivity technology will play a greater role of wireless carrier aggregation network between different systems in the future.