 for AUL-enabled HARQ processes, transmit power command, and transmit PMI [5,6]. LTE Rel-13 introduced UE RSSI measurements with configurable measurement granularity and time instances of the reports which can be used for the assessment of hidden nodes by an eNB which is located near specific UEs. For example, if UE measurement shows a high RSSI value when the serving cell is inactive due to LBT, it can imply the presence of Operation in Unlicensed and Shared Spectrum 871 hidden nodes and can be considered for channel (re) selection. The DFS is a regulatory requirement for certain frequency bands in some regions, for example, to detect interference from radar systems and to avoid cochannel operation with these systems by selecting a dif- ferent carrier on a relatively slow time scale. The corresponding time scales for DFS are in the order of seconds and can, therefore, be at an even slower time scale than carrier selec- tion. This functionality is an implementation issue and does not impact the specifications. As we mentioned earlier, LBT procedure is defined as an algorithm by which a UE per- forms one or more CCAs prior to transmitting on the channel. It is the LAA equivalent of the DCF and EDCA MAC protocols in Wi-Fi (see Fig. 8.13). 3GPP has specified four categories of LBT as follows: Cat-1: no LBT Cat-2: LBT without random backoff Cat-3: LBT with random backoff and fixed CW Cat-4: LBT with random backoff and variable CW A simple approach to ensure coexistence with Wi-Fi is to make the LAA LBT procedure for both data and discovery reference signal 9 as similar as possible to IEEE 802.11 DCF and EDCA protocols. This was the underlying design principle in Rel-13 LAA LBT mecha- nism for detecting the presence of Wi-Fi which can be summarized as follows: An LAA-enabled node must sense the carrier to be idle for a random number of 9 us CCA slots prior to data transmission. If the energy in a CCA slot is sensed to be above an energy detection (ED) threshold, then the process is suspended and the counter is stopped