## 1 Introduction
It is clear that introducing LTE in the unlicensed spectrum as is will cause significant coexistence issues with other well-established technologies such as Wi-Fi, IEEE 802.15.4 or Bluetooth. This means that LTE will have a negative impact on the performance of traditional unlicensed technologies in terms of throughput, latency and other Quality of Service (QoS) guarantees [6], affecting their  
Figure 1: Deployment of LTE in the unlicensed spectrum next to the current infrastructure. applications such as wireless (sensor) networks, Device-to-Device (D2D) and Machine-to-Machine (M2M) communications. To this end, research has focused on the design and evaluation of coexistence techniques for LTE, in order to enable fair spectrum sharing with other technologies operating in the unlicensed spectrum, and in particular with Wi-Fi. On the other hand, much less attention has been paid to cooperation techniques between the two technologies. The networks that participate in a cooperation scheme are able to exchange information directly or indirectly (via a third-party entity) in order to improve the efficiency of spectrum usage in a fair way. In this article, we distinguish two different classes of cooperation between LTE and Wi-Fi, and for each class, we propose and analyze potential cooperation techniques that can be applied. For each cooperation technique, we analyze the advantages and disadvantages regarding the design and deployment complexity, the flexibility and the efficiency they could offer. The proposed techniques can contribute to the open discussion regarding the standardization process of the LTE operation in the unlicensed spectrum. The main contribution of this work is summarized as follows:  
* Classification of techniques that can be applied on co-located LTE and Wi-Fi networks
* Detailed analysis of the current state of the art regarding LTE in the unlicensed spectrum and Wi-Fi covering:
* Analysis of the standard LTE and Wi-Fi protocols
* The regional regulations for the unlicensed spectrum
* The impact of LTE on Wi-Fi without applying any coexistence technique
* The current approaches for coexistence between LTE and Wi-Fi
* Analysis of the different concepts of cooperation between LTE and Wi-Fi and potential techniques that can be applied for realizing each concept
* Comparison and feasibility of the different presented concepts  
The remainder of the article is organized as follows. Section 2 presents a classification of the techniques that can be applied when LTE and Wi-Fi networks are co-located and operate in the same (unlicensed) frequency band. Section 3 discusses the current state of the art for LTE operation in the unlicensed spectrum. Next, in Section 4, we analytically present the concept of direct cooperation between LTE and Wi-Fi via in-band energy level patterns and showcase possible cooperation techniques. Section 5 presents the concept of cooperation between LTE and Wi-Fi using indirect communication through a third-party entity and describes possible cooperation techniques. In Section 6, we compare the proposed concepts and techniques. Finally, in Section 7, we conclude the paper and discuss plans for future work.