 a radar system is using the band (Dynamic Frequency Selection, DFS) or being able to coexist with other users of the band. The latter is often referred to as Clear Channel Assessment (CCA) or Listen-Before-Talk (LBT) and means that it is not always possible to transmit immediately if the intended channel is occupied. The maximum allowed transmission power also varies depending on the region and on the part of the 5 GHz band. Typically, some parts of the band are restricted to indoor use, with a maxi- mum transmit power of 250 mW or less. Other parts allow higher transmission powers, typically around 1 W, and consequently are also suitable for outdoor deployments. In some cases, such as the US 5.725-5.850 GHz, there are no specific requirements other than the transmission power limitation. 2G and 3G technologies can only operate on licensed frequencies. The same was true also for LTE in 3GPP Releases 8-12, while Release 13 added Licensed Assisted Access (LAA), which allows utilization of the 5 GHz unlicensed band together with some of the lower licensed frequencies with carrier aggregation. The unlicensed band can be utilized with different options, as shown in Figure 4.18. The first option is to use the unlicensed band for boosting downlink capacity and data rates on top of the licensed band. This solution is called LAA, where the primary cell is on the licensed band and secondary cells are on the unlicensed band, providing higher capacity and higher data rates. LAA rollout Licensed band needed. Licensed band needed. Standalone unlicensed Unlicensed for Unlicensed for downlink solution. No need for downlink. and for uplink licensed band. Licensed Unlicensed Licensed Unlicensed Licensed Unlicensed Figure 4.18 Options for utilizing unlicensed band in 5G. 4.5 Unlicensed Band started from the United States in 2017 and has been available in devices since 2018. LAA includes solutions to comply with the emission and coexistence rules of the 5 GHz band. The second option is to boost uplink data rates also by using unlic