 via HARQ feedback (NACKs) from UEs, then the CW is doubled for the next LBT (see Fig. 8.14). Once downlink transmission is complete, a new random backoff is chosen and used with the next transmission. A single, short CCA period of 25 us can be used to transmit control information without accompanying data such as DRS. This is aligned with the CCA duration used for Wi-Fi beacon frames. Rel-13 LTE defined an LAA equivalent to the four Wi-Fi priority classes in the form of four sets of minimum and maximum CW sizes, maximum channel occupancy times, and deferred period CCA slots. An LAA/Wi-Fi coexistence study suggests that the coexistence performance is more sensi- tive to factors that affect the channel occupancy (e.g., control signals) rather than to the 874 Chapter 8 Downlink SCell Update CW based on HARQ feedback and draw counter End of burst 0<N<CW update CW Defer Channel Channel Freeze period Post TX Backoff Short Uplink PCell report report Figure 8.14 An example of LAA downlink transmission with LBT CW updates based on HARQ ACK/NACK feedback. The UE provides HARQ feedback and CSI reports on the licensed carrier [17,23]. choice of parameters in the LBT CCA and backoff algorithms. Consequently, the coexis- tence is highly affected by the behavior of the upper-layer protocols; bursty traffic pattern; HARQ-based CW slot update; and either CTS-to-self or support for lower Wi-Fi energy detection thresholds, which seems to be a fundamental feature to be supported by LAA to allow coexistence with Wi-Fi and protect the LAA performance in the presence of hidden nodes [12]. To minimize interference to Wi-Fi transmissions, an LTE system can mute its operation in almost blank subframes (ABS). These subframes are called almost blank because LTE can still transmit some broadcast signals, control signals, and synchronization signals over these subframes. However, these signals only use a small fraction of system resources in the time/ frequency domain and reduced power. Therefore, cochannel interference is much less during t