For the universal mobile telecommunication system (UMTS) network, the long term evolution (LTE) has been specified for further enhanced data rates and less delay (see Non-Patent Literature 1). The LTE advanced has been specified for achieving even wider bands and higher speed than those of LTE, and the succeeding systems of LTE, such as future radio access (FRA), are under study.
The system band LTE Rel.10/11 includes at least one component carrier (CC) that uses an LTE system band as one unit. Band expansion through the aggregation of multiple component carriers is referred to as carrier aggregation (CA).
For LTE Rel.12, which is a more recent LTE system, various scenarios are under study to use a plurality of cells in different frequency bands (carriers). When essentially identical radio base stations are used for a plurality of cells, aforementioned carrier aggregation can be used. When completely different radio base stations are used for a plurality of cells, dual connectivity (DC) can be used.
LTE Rel.8 to Rel.12 have been specified assuming the exclusive use of frequency bands given to providers, i.e., licensed bands. For licensed bands, 800 MHz, 2 GHz, and 1.7 GHz, for example, are used.
LTE Rel.13 or later also target the use of frequency bands that do not require licenses, i.e., unlicensed bands. Unlicensed bands include 2.4 GHz and 5 GHz, which are the same bands as Wi-Fi. LTE Rel.13 aims at carrier aggregation of licensed bands and unlicensed bands (license-assisted access (LAA)) and will possibly aim at dual connectivity and stand-alone unlicensed bands in future.
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