anced PDCCH (EPDCCH) is covered more in Chapter 3 in connection with LTE-M introduction. 2.3.1 Carrier Aggregation The introduction of carrier aggregation (CA) in Release 10 [3] is the most successful LTE- Advanced feature and has been deployed already by several operators in the field. The principle LTE Small Cell Optimization Inter-band CA Band-spacing 20 MHz Duplex-spacing 20 MHz 20 MHz Uplink Downlink Carriers Intra-band CA 20 MHz Duplex-spacing 20 MHz 20 MHz Uplink Downlink Carriers Intra-band non-contiguos CA (Release 11) Carrier spacing 20 MHz Duplex-spacing 20 MHz 20 MHz Uplink Downlink Carriers in the same band Figure 2.10 Different types of carrier aggregation itself is relatively straightforward, with the UE now being able to receive up to five carriers (with downlink carrier aggregation) or able to transmit with up to five carriers in the uplink direction (with uplink carrier aggregation). There are three different types of carrier aggregation as illustrated in Figure 2.10. Inter-band carrier aggregation, when the UE received two carriers that are from different frequency bands. For example, 850 and 1800 MHz bands are aggregated in Korea, whereas in Europe, 1800 MHz is aggregated together with 2600 MHz band. Intra-band carrier aggregation is usable when one operator has more than 20 MHz bandwidth available from the same frequency band. Intra-band carrier aggregation was extended for intra-band on-contiguous case in Release 11. This is needed in some countries where an operator has allocation in one band but the allocation for some reason is not continuous but there is another operator or other systems in between. From the radio protocol point of view, carrier aggregation mostly impacting the MAC layer as layers above MAC do not see a difference from Release 8-based operation, with the exception of higher peak user data rates being enabled. The MAC layer scheduler in the eNodeB needs to consider the CQI feedback from UE to determine which carrier is best suited, in addition to network load situation an