## 7 Conclusions and Future Work  
Towards 5G, the number of HetNets is expected to increase rapidly. These HetNets consist of different well-established wireless technologies that operate next to each other. Each of these technologies has its own user target group, as it is suitable for specific applications (sensor networks, D2D communications, M2M communications, etc.). Among them, Wi-Fi is the most popular and widely-used wireless technology in the unlicensed spectrum. Recently, LTE in the unlicensed spectrum has been introduced, as it is a technology that can play an important role in dealing with the tremendous wireless traffic increment. Hence, scenarios in which co-located LTE and Wi-Fi networks operate in the same band will soon become very common. Based on this fact, the research community needs to look into cooperation techniques among different technologies in order to use the wireless spectrum as efficiently as possible.  
In this article, we describe different cooperation techniques that can be applied between co-located LTE and Wi-Fi networks. These techniques are classified into two main categories. According to the first one, the networks cooperate directly by sending, receiving and interpreting in-band special patterns. In the second category, the cooperating networks can intercommunicate indirectly using a third-party entity such as a CCE. Each technique requires different implementation effort and offers different cooperation flexibility and spectral efficiency. Subsequently, for each proposed technique we analyze the open issues and challenges, as well as the required changes to the LTE and Wi-Fi protocols taking into account regional regulations.  
The concepts that are described in this article will be used as a cornerstone for our future work. In the near future, this work can be extended towards the implementation and the comparison of the proposed techniques by initially performing simulations according to the performance indicators as they are mentioned in Table 3. Further, implementation and evaluation based on real hardware can also be done using the LTE and Wi-Fi infrastructure of the W-iLab2 testbed at imec. This way, each cooperation technique can be examined in detail and the analytical results of the provided fairness and spectral efficiency can be obtained. This work can further contribute to the ongoing research and standardization towards an efficient and fair spectral sharing between LTE and Wi-Fi.  
The research leading to this article has received funding from the European FP7 Program under Grant Agreement 612050 (FIRE LTE testbeds for open EXperimentation (FLEX) project) and from the European H2020 Programs under Grant Agreements 688116 (elastic WIreless Networking Experimentation (eWINE) project) and 645274 (Wireless Software and Hardware platforms for Flexible and Unified radio and network controL (WiSHFUL) project).  
All the authors contributed to the work presented in this article. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] organized the work, carried out the research study and reviewed the paper. [PERSON] assisted in revising and restructuring the paper.  
The authors declare no conflict of interest.