arrier Aggregation Release 12 contains an important building block for bringing together FDD and TDD operation modes, namely FDD-TDD carrier aggregation. It allows to extend the carrier aggregation framework in such a way that PCell could be on an FDD band and SCell could be on a TDD band, or also other way around, as shown in Figure 3.6. In Release 10-based carrier aggregation, both bands have to be FDD bands or TDD bands. In most cases, the FDD band LTE Small Cell Optimization Figure 3.6 FDD-TDD carrier aggregation is a lower band in frequency thus more likely to be used as PCell, thus in the first phase, the interest for practical combinations, such as 800 MHz with 2.6 GHz TDD naturally leads towards FDD PCell and TDD SCell. From an operator point of view, the use of FDD-TDD carrier aggregation makes it easier to utilize both paired and unpaired bands. If an operator has already aggregated, for example, two FDD bands of 20 MHz each, the UE can reach 300 Mbps downlink peak data rates using carrier aggregation. If now there is only a single TDD band available (with 20 MHz), then using only TDD for the connection would mean smaller achievable peak data rate even if the actual network capacity would be increased. This would lead to such a situation in the small cell environment (assuming TDD used with small cells at higher frequency without carrier aggregation) that an operator would provide worse user experience with small cells than with macro cell environment, at least with low load conditions. Most of the carrier aggregation principles are valid with FDD-TDD aggregation. Mainly some changes were needed especially for the handling of Ack/Nack signalling, especially if TDD band would be the PCell as then the uplink resources are not always available. From the practical deployment point of view, the most cases currently on the table are such that TDD band is clearly at higher frequency thus that becomes naturally the capacity extension band while coverage is ensured with the FDD band, as shown in Figure 3.7. FDD/