## 5 Indirect Cooperation via a Third-Party Entity
### Negotiation between LTE and Wi-Fi  
The method described in the previous section gives to the coordinator the possibility to decide, in the beginning of a time frame, the duration of the LTE transmissions and the following mute period. These durations remain stable during the whole time frame duration. In a different approach, the CCE can schedule variable durations of LTE transmission and mute periods in the same time frame. This technique can offer higher transmission flexibility and can better serve the QoS requirements as the networks can access the medium in a more dynamic way. The proposed methodology is illustrated in Figure 15.  
Figure 14: Flowchart of LTE and Wi-Fi transmissions adjustment by Central Coordination Entity (CCE).  
As can be seen, the time domain is divided into time frames with a range from tens of milliseconds up to a few seconds. In the beginning of each frame, there is a negotiation phase during which the participant networks of the cooperation scheme report their requirements to the coordinator. As presented in Section 5.3, the coordinator keeps a record of the channel activity. Hence, it knows the percentage that each network has occupied the channel in the past. When the coordinator receives the requirements during the negotiation phase, it divides the time frame into slots and assigns them to the different networks based on their demands and the history of the channel activity. The proportion of the current requirements and history will be decided in an advanced way, similar to the one described in Section 5.3. In this technique, the wireless technologies access the medium in an inter-technology TDMA way. When a timeslot is assigned to an LTE network, the eNB schedules resources for the UEs that are attached to it. On the other side, if a timeslot is assigned to a Wi-Fi network, then the Wi-Fi nodes are competing for the medium using the traditional CSMA/CA method. Before a transmission, both LTE and Wi-Fi networks must perform a channel estimation to ensure that the channel is free from potential ongoing transmissions by other networks that do not participate in the cooperation mechanism. If the channel is sensed as busy, then the network continues to sense the wireless medium till the end of the assigned slot. If the medium becomes idle, the corresponding network starts a transmission for the remaining time. The CCE can take into account such potential cases for future scheduling decisions in order to assign longer or more slots to the network that missed one or more assignments during the previous frame(s).  
In this technique, the coordinator needs to ensure synchronization of the participating networks in a frame time domain. This way, the networks will be able to express their requirements and negotiate about the spectral requirements during the negotiation phase. Additionally, the participating networks will be able to access the corresponding assigned LTE and Wi-Fi time slots that the scheduler creates in a synchronized manner.  
Such a cooperation mechanism requires modifications of LTE and Wi-Fi, so they can express their requirements during the negotiation phase. Moreover, the networks should be able to interpret the messages sent by the CCE and transmit only during the timeslots that have been assigned to them by the coordinator. Additionally, a sophisticated CCE needs to be implemented, which will be able to  
Figure 15: Negotiation phase between LTE and Wi-Fi and timeslot assignment.  
receive the requirements from different networks and to assign timeslots using advanced methods, taking into account the current requirements and the history of the channels' occupancy.