## 5 Indirect Cooperation via a Third-Party Entity  
### Introduction  
The previous section described different cooperation techniques, wherein the participating LTE and Wi-Fi networks can cooperate directly by sending, sensing and interpreting in-band energy patterns. This section goes one step further and discusses the cooperation possibilities, when LTE in the unlicensed spectrum and Wi-Fi are able to exchange messages and express their requirements through a third-party entity. This third-party entity can communicate with both technologies in order to exchange the necessary information that can lead to optimal spectrum usage and enhance the user experience. The third-party entity can be either a central entity, such as a Central Coordination Entity (CCE), or it can be deployed in a distributed manner (Distributed Coordination Entity (DCE)). In the DCE case, a third-party entity must be connected with the base station of each network (LTE eNB or Wi-Fi AP). Thus, the different entities must communicate with each other in order to convey the messages from one network to the other. In this case, the complexity of the cooperation schemes increases. For this reason, in the rest of the article we focus on the usage of the CCE as a third-party entity. However, for the proposed techniques, both a CCE or a DCE can be used.  
It is expected that this kind of cooperation can lead to better spectral management results compared to the previously-mentioned techniques in Section 4, as both technologies can explicitly declare their requirements and speak indirectly to each other through the CCE. Such a type of cooperation between LTE and Wi-Fi can offer high flexibility, which can lead to the implementation of advance cooperation techniques. Moreover, cooperation of this type does not increase the complexity of the operation of the LTE and Wi-Fi transmitter and receiver. However, the implementation of the CCE, as well as the requirement for the CCE entity as part of the network adds extra complexity to the design of such a system.  
The Wi-Fi APs and the LTE eNBs can be connected to the CCE by either a wired or a wireless link. In the case of a wired connection, the on-demand communication among the networks and the CCE is guaranteed. Of course, wired connectivity limits the flexibility in terms of deployment. These deployment scenarios are limited mainly to indoor deployments, such as in office environments. On the contrary, wireless communication offers higher deployment freedom, but the transmission of the messages between the CCE and the wireless technologies has to deal with the same well-known issues that arise in every wireless communication, such as interference. For simplicity, in the rest of the paper, we assume that there is wired communication among the CCE, the eNBs and the APs.  
Furthermore, the CCE must ensure synchronization between the networks that participate in the cooperation scheme. The synchronization among the networks for such cooperation techniques is often critical, as the exchange of information and the access to the medium need to be done in precise time instances. The synchronization requirements depend on each single technique, and therefore, we discuss them in the following subsections.  
The rest of this section analytically describes different possible cooperation methodologies that can be applied, when a CCE is available among LTE and Wi-Fi. In addition, for each method, we discuss the changes that are required for both LTE and Wi-Fi protocols.