3.1 Basic Concepts
International Mobile Telecommunications Advanced (IMT-Advanced) requirements establish a minimum support of 1 Gbps and 100 Mbps peak rates for low-mobility and high-mobility users, respectively. In order to fulfill these challenging requirements, wider channel bandwidth than legacy Third Generation (3G) systems has to be supported. However, the available spectrum resources of operators differ depending on the specific country, being spread out over different frequency bands and with different bandwidths. Therefore, all IMT-A technologies foresee one of their key features as the aggregation of continuous or discontinuous spectrum in order to achieve wider bandwidth and consequently increase transmission capability. This concept is usually known as Carrier Aggregation (CA) or spectrum aggregation.
Discontinuous CA has the advantage of having spectral diversity gain, due to the use of different frequencies, which implies different types of fading channels. However, this requires having several physical layer processing chains – one per each of the aggregated carriers. On the other hand, contiguous CA can save much spectrum because many subcarriers used as guard bands can be employed for data and control information. In the absence of carrier aggregation some frequency chunks became unused whereas with carrier aggregation these guard bands could be removed.