Long Term Evolution (Advanced) - LTE(-A)
Carrier aggregation

Even if LTE-A according to Release 10 reaches higher spectral efficiency comparing to former releases of LTE, the maximum peak rate of 1 Gbps required by IMT-Advanced for 4G mobile networks cannot be reached by using conventional frequency bands with width of up to 20 MHz. To meet this requirement, bandwidth must be increased. Therefore, LTE-A enables to merge radio resources across multiple bands (carriers) and perform parallel transmission to a UE. This approach is known as carrier aggregation. This technique is an enhancement of DC used in UMTS where only up to two carriers can be merged. In LTE-A, each carrier used for data transmission is referred to as a component carrier. LTE-A enables to use up to five component carriers in downlink and five in uplink for a transmission. It results in using bandwidth of up to 100 MHz. In both downlink and uplink, one component is denoted as a primary component and all other are secondary components. It means, each UE has to have one primary component and no or several (up to four) secondary components. The primary component is used for permanent signalling in idle mode (it is battery saving mode in case of no data communication). The secondary components are exploited for increase the bandwidth for data communication but not for signalling in idle mode. Assignment of primary and secondary components is UE specific, that is, it can vary from UE to UE.

There is no need for contiguous carriers to be assigned to a UE. Three types of carrier aggregation are enabled as shown in figure:

- Intra-band aggregation with contiguous components - this type is the easiest way of carrier aggregation since the band can be seen as one component from the UE point of view. Therefore, no special requirements are implied on either UE or HeNB (no additional transceivers or receivers are required).
- Intra-band aggregation with non-contiguous components - the signal cannot be seen as one transmission, thus more transceivers/receivers are required. This increases the cost of equipments.
- Inter-band aggregation (with non-contiguous components) - again, more transceivers/receivers are needed due to separation of the carriers in frequency.

Types of carrier aggregation

All UEs with carrier aggregation support can access all components in parallel. If a UE does not support carrier aggregation, it can access each carrier individually in conventional way of LTE to ensure backward compatibility.

The carrier aggregation is available for TDD as well as for FDD modes but the same mode must be applied for all components used by a UE. In addition, the same configuration of uplink and downlink subframes must be kept for TDD while configuration of special subframes can be different for individual components.