sary, specific for the unlicensed band operation is to be transmitted using the 5 GHz band. For example, the Release 8-based broadcast channel is not necessary to support over the 5 GHz band as it can be always obtained via the primary cell on licensed band. The common reference signals (CRS) cannot be transmitted continuously like in Release 8-based LTE Small Cell Optimization CCA detects CCA detects channel is busy channel is free Fixed frame period Fixed frame period Channel occupancy time Idle period Idle period with CCA with CCA Short control signalling (SCS) Figure 10.4 Short control signalling principle LTE operation, but the operation needs to be more dynamic in that respect as well. Simi- larly, the sounding reference signals (SRS) in the uplink are not likely to be used in their current form. 10.6 LTE Principle on Unlicensed Band The LTE operation on the unlicensed band is built on top of the LTE-Advanced carrier aggregation, which has been deployed commercially since 2013. The simplest form of LTE- Unlicensed would be to use the unlicensed band with downlink-only carrier aggregation, while the use of uplink would also be possible in-line with 3GPP carrier aggregation principles, as illustrated in Figure 10.5. This is similar to the first-phase LTE-Advanced carrier aggregation in the commercial networks which have started with the downlink-only aggregation. The primary cell ensuring connection maintenance is always located on the licensed band carrier. When operating with downlink only on the unlicensed band (a.k.a. supplemental downlink), the LTE eNodeB can perform most of the necessary operations to ensure reliable communi- cations, including checking whether the intended unlicensed channel is free from other use. The LTE terminal capable of operating on the unlicensed band needs to be able to make the necessary measurements for support of the unlicensed band operation, including providing the feedback when the terminal is in the coverage area of an LTE eNodeB transmitting with Supplemental downlink C