uous transmission on a carrier with limited maximum transmission duration, DFS for radar avoidance in certain bands, and multi-carrier transmission across multiple unli- censed channels. The DTX and LBT have had major impacts on various aspects of LTE functionalities including downlink/uplink physical channel design, channel state informa- tion (CSI) estimation and reporting, HARQ operation, and radio resource management. In 5 GHz band, about 500 MHz of unlicensed spectrum is available for LAA use. This unli- censed band can be divided into multiple channels of 20 MHz bandwidth. The selection of LAA carrier(s) with minimal interference is the first step for an LAA node to coexist with Wi-Fi systems in the unlicensed spectrum. However, when a large number of nodes are present, interference avoidance cannot be guaranteed through channel selection, thereby, sharing carriers between different technologies is required. The carrier selection can be further performed periodically by adding or removing unlicensed carriers as required. These carriers are then configured and activated as secondary cells (SCells) for use by LAA-enabled UEs. The 3GPP NR has initiated a study item in Rel-16 to investigate the required amendments in the Rel-15 NR to extend the operation to the unlicensed bands in sub-6 GHz and above 6 GHz. Meanwhile, the LTE track has continued to enhance the LAA and non- homogeneous LTE/Wi-Fi carrier aggregation solutions, which global operators have already started to deploy. In this chapter, we will review the general aspects and use cases of the operation in unlicensed spectrum including identifying the unlicensed frequency bands, overview of IEEE 802.11 operation, which is important for understanding the coexistence studies, as well as the necessary NR and LTE enhancements to address the unlicensed band operation from various aspects, including network architecture, radio access protocol struc- ture, and L1/L2 processing, as well as deployment and implementation considerations. 8.1 General Aspects and Use