## 3 State of the Art
### Proposed Coexistence and Coordination Techniques
#### Other Related Work
This protocol enhances the coexistence with Wi-Fi and increases the overall system performance. The protocol consists of two different mechanisms named on-off adaptation for channel occupancy time and short-long adaptation for idle time. The first mechanism is responsible for adapting the channel occupancy time of LTE based on the load of the network, while the second one adapts the idle period based on the Contention Window (CW) duration of Wi-Fi. The authors in [25] propose an LBT mechanism for LTE LAA that aims to share the medium in a fair way towards the increase of the overall system performance. This work includes both a mathematical analysis and a validation via simulation of the proposed LBT scheme. The results show that a proper selection of LAA channel occupancy and the backoff counter can increase the performance of Wi-Fi. In [26], the authors study the coexistence among LTE LAA and Wi-Fi using the LBT category four-channel access scheme. The behavior of LAA eNB is modeled as a Markovchain, and the obtained throughput is adopted as the performance metric. The proposed LBT scheme uses an adaptive CW size for LTE LAA. According to the results, the proposed scheme outperforms the fixed CW size. In [27], the authors examine how LTE cells in the unlicensed spectrum from different operators can adjust their CW in order to tune the LBT algorithm and provide coexistence both with Wi-Fi and among themselves in an altruistic way. The interaction of LTE cells in the unlicensed spectrum is studied using a coalition formation game framework, which is based on the Shapley value. In [28], the authors present an analytical model for evaluating the performance of coexistence between LTE and Wi-Fi. This model has been used to obtain baseline performance measures. The results of the model have been partially validated via experimental evaluation using Universal Software Radio Peripheral (USRP) platforms. Moreover, the authors propose an inter-network coordination with logically centralized radio resource management across LTE and Wi-Fi as a solution to improve coexistence. The authors in [29] propose two non-coordinated and two coordinated network management approaches to enable coexistence. Regarding the non-coordinated techniques, the first one proposes eNB to perform LBT on different channels and to switch to a different channel after a transmission, while the second proposes LTE to offer transmission opportunities of variable duration to Wi-Fi after a transmission based on the occupancy of the medium. Concerning the coordinated methodologies, the first one proposes a Network Function Virtualization (NFV) interconnection to combine the Wi-Fi network and the service provider of LTE in the unlicensed spectrum. This way, channel selection and seamless transfer of resources between the two technologies can be enabled, using the in-the-cloud control of distributed APs. The second method proposes the management of coexistence using the X2 interface among the eNBs. The eNBs can exchange information and schedule ABS in different subframes, thus giving more opportunities to any Wi-Fi network that is located potentially within their proximity. In the aforementioned schemes, the different RATs are under the control of the same mobile operator. The coordination between the wireless technologies targets the enhancement of the overall QoS that the operator offers (e.g., perform load balancing via frequency coordination). Finally, the authors in [30] provide a detailed survey of the coexistence of LTE and Wi-Fi on 5 GHz with the corresponding deployment scenarios. They provide a detailed description of the coexistence-related features of LTE and Wi-Fi, the coexistence challenges, the differences in performance between the two different technologies and co-channel interference. They extensively discuss the proposed coexistence mechanisms between LTE and Wi-Fi in the current literature.