 802.1.ac; thus some more flexibility could be potentially provided here. While the LTE downlink is well suited for this type of requirement, the uplink structure is likely to require some more changes to satisfy the minimum bandwidth occupancy requirements. Adaptive channel access is best known as listen-before-talk (LBT) or clear channel assess- ment (CCA). The key principle is to determine first if the channel is used by other users of the band before transmitting on the channel. The European regulation defines energy threshold, which is dependent on the transmit power as well, to be met before the channel can be considered 'free'. For the operation, there is need to stop the transmission and to check the channel status after the maximum channel occupancy time has been exceeded. This allows other systems to coexist with other users (other than radar) on the band. This may result in a situation that the particular channel used earlier may not be available as expected due to other increased activity of the band. Of course, all the other systems will need to follow the same LBT/CCA principles but by nature the unlicensed band may have varying availability due to the load from other system. This aspect has clear impacts on the LTE specifications and is analyzed more in Section 10.5. The allowed transmission power also varies depending on the part of the band. Typically, the lower portion of the 5 GHz band is often determined and allowed for indoor-only use with 200 mW or less transmit power while the upper part of the spectrum enables higher transmission power, typically 1 W at the antenna connector. 10.3 Operation Environment The use of LTE on unlicensed bands has been generally considered for public indoor cells or outdoor hot spots, places where there is coverage from licensed band LTE operation but additional capacity would be beneficial, as shown in Figure 10.2. The intention of the 3GPP ongoing work is not to define a stand-alone system that could be used, for example, residential/home environments since the