itically affect the performance of WiFi networks. Initially they only considered the downlink operation LTE-A (LTE Advanced) Carrier Aggregation (CA) in the unlicensed band. This was later expanded to include the simultaneously operate downlink and uplink [146]. The LTE LAA employed a Listen Before Talk (LBT) mechanism to avoid collision and interference between users. LTE-U is another option for operating LTE in an unlicensed spectrum, where LTE base stations exploit transmission gaps to facilitate coexistence with WiFi networks. The devel- opment of LTE-U technology is led by the LTE-U Forum, an industry alliance. LTE-U has been designed to operate as an unlicensed LTE in countries where the LBT technique is not mandatory, such as the United States and China. LTE-U defines the operation of pri- mary cells in a licensed band with one or two secondary cells (SCells), every 20 MHz in the 5 GHz unlicensed band: U-NII-1 and/or U-NII-3 bands, spanning 5150-5250 MHz and 5725-5825 MHz, respectively [146]. 10.2.3.1 ML-Based LBT Methods According to LTE LAA standards in 3GPP Release 13, the LTE system must perform the LBT procedure (also known as Clear Channel Assessment, CCA) and sense the channel prior to a transmission in the unlicensed spectrum. As shown in Figure 10.5, when the channel is sensed to be busy, the LTE system must defer its transmission by performing an exponential backoff. If the channel is sensed to be idle, it performs a transmission burst with a duration from 2 to 10 ms, depending on the channel access priority class [124]. To exploit the benefits of communications in an unlicensed spectrum using LTE-LAA, a DL approach for the resource allocation of LTE-LAA small base stations (SBSs) was proposed in [28]. The proposed method employs a proactive coexistence mechanism that enables future delay-tolerant LTE-LAA data requests to be served within a given prediction LAA burst LAA burst LTE LAA LAA CAA LAA CAA reports medium as reports medium as Packet WiFi CAA WiFi CAA reports medium as reports medium as 