## 4 Direct Cooperation via In-Band Energy Level Patterns
### Enhanced CSAT  
One negative aspect of the CSAT algorithm, as described in Section 3.4.2, is that it requires a very long sensing period ranging from 20 ms up to 100 ms, in order to observe the activity in the medium and decide the LTE \"ON\" and LTE \"OFF\" periods. Furthermore, at the end of an LTE \"ON\" period or at the end of a Wi-Fi opportunity slot during the LTE \"ON\" period, LTE starts transmitting without sensing the medium for ongoing Wi-Fi transmissions. This results in several collisions among LTE and Wi-Fi. These drawbacks can be eliminated by the use of an energy level pattern periodically transmitted by the eNB. Such a pattern can be sensed by Wi-Fi and other LTE networks in order to achieve inter- and intra-technology synchronization and to adjust their behavior. In the proposed methodology, we define three different energy level patterns that can be used for different purposes. These patterns are defined as follows:  
* Synchronization pattern that enables inter- and intra-technology synchronization and that is transmitted by the first activated network
* LTE identification pattern that is transmitted by the eNB of a newly-activated LTE network in order to inform the rest of the networks about its presence
* Wi-Fi identification pattern that is transmitted by the AP of a newly-activated Wi-Fi network in order to inform the rest of the networks about its presence  
Additionally, we define a new time frame as is depicted in Figure 8. This time frame starts with a period T\({}_{\text{SYNC}}\) dedicated to transmission and reception of the synchronization pattern. Then, the main part of the frame is called T\({}_{\text{TX}}\) and is divided into slots for LTE and Wi-Fi traffic. The last part of the frame is called T\({}_{\text{IDENT}}\), and it is used for LTE and Wi-Fi pattern transmission, in order to identify any new networks and adjust the LTE and Wi-Fi slots for the next time frame. Initially, when a new network is activated, it must sense the medium for a period of time equal to a frame in order to discover potential synchronization patterns. If such a pattern does not exist, then the network starts periodically transmitting a synchronization pattern signal to enable inter- and intra-technology synchronization. On the other hand, if the new network senses a synchronization pattern, then  
Figure 7: LTE identification via the predefined energy level pattern. it does not initialize a periodic synchronization pattern signal transmission, but it keeps sensing the medium to identify the next synchronization pattern that is expected at the beginning of the next frame. At the moment that two sequentially synchronization patterns are sensed, then the new network can be in synchronization with the rest of the networks. The length of a frame can be stable over time or can vary based on the number of cooperating networks and the amount of transmitted traffic. In the case that a variable frame size is used, then the new network must sense the medium to discover a potential synchronization pattern for a time period that is equal to the maximum frame length. From the moment that the newly-activated network is in synchronization with the rest of the networks, it must transmit a corresponding LTE or Wi-Fi identification pattern during the next TIDENT period. This way, the rest of the networks will be notified for the new LTE or Wi-Fi network. The newly-activated network can identify the LTE and Wi-Fi slots that have already been created by sensing the medium during a TTX period. The network that is in charge of transmitting the synchronization pattern transmits its identification pattern only after it senses the first identification pattern from another network during the same TIDENT period. If an LTE or Wi-Fi identification pattern is sensed during the TIDENT, then the LTE and Wi-Fi slots during the TTX period are readjusted. This readjustment is done based on the number and the type of the co-located networks.