s illustrated in Fig. 8.39. In this figure, different shades represent different frequencies in the outer sectors where the same frequency is used in the inner sectors. Deterministic channel sharing relies on LTE centralized scheduling to periodically turn off its transmission SO that Wi-Fi users can access the shared channel. Among time-domain coexistence mechanisms, we have discussed CSAT and blank-subframe allocation. A blank- subframe is an LTE subframe where transmission is muted SO that Wi-Fi users can access the channel. Similar to CSAT, a blank-subframe allows time-domain sharing between LTE unlicensed and Wi-Fi networks. In each radio frame, the eNB can configure a certain 908 Chapter 8 Sector 1 Sector 6 Sector 2 Inner sector 1 Sector 5 Sector 3 Sector 4 Figure 8.39 Illustration of LTE ICIC scheme where cell-center and cell-edge users have frequency reuse factor of 1 and 3, respectively. Radio frame (10 ms) LTE on subframes Blank subframes Figure 8.40 Example of blank-subframe allocation in an LTE radio frame [25]. number of blanked subframes based on the measurement of Wi-Fi's traffic load (see the example in Fig. 8.40). Fairness can be achieved by adjusting the number of blank subframes in each radio frame. Blank subframe offers more flexibility than CSAT as the ratio between the non-blank and blank subframes can be dynamically adjusted at the frame level, which is shorter than a CSAT cycle. Moreover, the positions of these blank subframes in each frame do not need to be consecutive. A blank-subframe is similar to the ABS used for enhanced ICIC (eICIC) mechanism in LTE-based heterogeneous networks. An ABS is a subframe Operation in Unlicensed and Shared Spectrum Opportunistic air time Opportunistic air time of Wi-Fi before LTE of Wi-Fi after LTE lowers Tx power lowers Tx power LTE Tx power (before) LTE Tx power (after) Wi-Fi energy detection threshold Time (ms) Figure 8.41 An example of Wi-Fi transmission opportunities before and after LTE power reduction [25]. during which only control and reference s