## 5 Indirect Cooperation via a Third-Party Entity
### Adjustment of LTE Transmission Based on Wi-Fi Requirements  
As discussed in Section 3.4.1, LTE LAA, in its current form and after a successful CCA, starts transmitting for a duration of 2-10 ms, depending on the channel access priority class to which it belongs (Table 2). Thus, the duration of an LTE transmission is much longer than a typical transmission duration of Wi-Fi (typically a few hundreds of \(\upmu\)s), which transmits only one packet after a successful CCA. In order to balance LTE occupation of the wireless medium versus Wi-Fi, an event-based coordination technique can be applied, which adjusts the LTE transmission duration based on the requirements of Wi-Fi. Additionally, a mute period at the end of an LTE transmission is introduced, as is depicted in Figure 10. Wi-Fi can exploit this period to gain additional access opportunities to the medium resulting in a longer time period of Wi-Fi transmissions. The proposed scheme requires that both LTE and Wi-Fi perform a CCA before a transmission. This guarantees that the regional regulations are satisfied and that the cooperation scheme is able to coexist with other potential co-located legacy networks. Figure 12 illustrates the proposed network architecture. Initially, when a new Wi-Fi network is activated, the AP collects the requirements of the Stations (STAs) that are connected to it and sends them to the CCE. The AP keeps collecting the STAs requirements periodically. Typically, the requirements of a network change on a relatively slow timescale. Thus, this period can be in terms of tens of milliseconds up to a few seconds. When the AP collects new requirements, it checks the variation compared to the previous state reported to the CCE. If this variation exceeds a defined threshold, then the AP reports the new requirements to the CCE. The Wi-Fi AP's report to the CCE may consist of two different types of information. The first one comprises the summarized statistics of the load of each STA. The second type refers to the future  
Figure 12: Adjustment of LTE transmission based on Wi-Fi requirements. requirements for the next time frame, reported by the STA during the current time frame. These future requirements correspond to a summarized bit rate per second for every Wi-Fi Multimedia (WMM) Access Categories (ACs). For the 802.11 standard, four different WMM priority classes are defined for handling the data traffic regarding the QoS requirements. These classes are the following:  
* AC_BK (background traffic)
* AC_BE (best effort traffic)
* AC_VI (video traffic)
* AC_VO (voice traffic)  
After the CCE receives the new requirements, it evaluates them, and if a change of LTE transmission behavior is needed, it triggers an event and informs LTE about the new transmission duration and the new length of the mute period. Hence, the next LTE transmission will be done according to the new configuration. In the case of multiple LTE networks, the transmission duration and mute period should be synchronized among them. This way, a scenario according to which an LTE transmission occurs during the mute period of another LTE network occupying the Wi-Fi grant can be avoided. This way, under a heavily loaded Wi-Fi network or if Wi-Fi needs to transmit delay-sensitive traffic, the coordinator schedules a shorter transmission duration for LTE and/or a longer mute period after a transmission. The ratio of LTE transmission duration and LTE mute duration must be chosen carefully, so that LTE does not suffer from continuous short transmission durations and long mute timeslots in the unlicensed spectrum. Hence, CCE must keep a record of the last configurations of the LTE transmission and mute duration over time. This way, it will be able to maintain a balance between serving the Wi-Fi requirements and giving equal channel access opportunities to all of the cooperating networks.