## 1 Introduction  
Over the past few years, the technological growth combined with the proliferation of wireless devices such as sensors, smartphones, laptops and wearable technology has changed the way that information is exchanged. The number of interconnected devices and the number of Heterogeneous Networks (HetNets) increase rapidly. The development and the consolidation of wireless sensor networks has further contributed to the increase of the wireless traffic, as often they consist of hundreds to thousands of wireless sensor nodes. The first-generation Internet has evolved into the Internet of Everything, where massive amounts of information are exchanged between devices using different types of mainstream and well-established wireless technologies such as LTE, Wi-Fi, IEEE 802.15.4 and Bluetooth. Recently, the sub-gigahertz bands have been extensively exploited by wireless technologies that offer wide ranging communications, such as LORA, SIGFOX and 802.11 ah. Moreover, high frequency bands such as mmWave are also being used for multi-gigabit speeds (IEEE 802.11 ad). According to Qualcomm, the amount of wireless traffic is expected to further increase by a factor of 1000 by 2020 [1]. Additionally, Cisco's latest forecast expects that the traffic from wireless and mobile devices will exceed the overall wired traffic by 2019 [2]. Based on these predictions, the wireless network capacity will soon become a bottleneck for the massive growth of wireless traffic. The 5G community has already started investigating solutions that can lead to the 1000\(\times\) challenge. These solutions include among others, enhanced massive Multiple-Input Multiple-Output (MIMO), carrier aggregation, higher-order modulation schemes such as 64-Quadrature Amplitude Modulation (QAM) or 256-QAM, cloud computing services and advanced network architecture modifications. At the same time, the adoption of LTE from different applications gains ground, as it is a technology that approaches the Shannon limit and can contribute significantly to solving the network capacity challenge. Recently, key players of the mobile world have proposed standards to the 3 rd Generation Partnership Project (3 GPP), which allow LTE operation in the unlicensed spectrum. To this end, 3 GPP announced the operation of LTE Licensed-Assisted Access (LTE LAA) [3], as an enhancement within 3 GPP LTE Release 13. LTE LAA will allow operators to use a secondary cell operating in the unlicensed spectrum, alongside the primary cell operating in the licensed band they own. The carrier aggregation that has been introduced in 3 GPP LTE Release 10 [4] will be used to enable this feature. There are two predominant proposals for LTE LAA. According to the first one, the secondary cell will operate in the unlicensed spectrum for supplemental Downlink (DL) traffic only, while Uplink (UL) traffic will be transmitted over the operator's licensed spectrum. In the second proposal, the secondary cell operating in the unlicensed spectrum can be used for both DL and UL LTE traffic. On the other hand, leading wireless stakeholders other than mobile operators are taking the first steps towards exploitation of LTE in the unlicensed spectrum as a standalone wireless solution complementary to Wi-Fi. To this end, they formed the MulteFire Alliance [5]. Their target is to decouple LTE from the operators, so it can be deployed by Internet service providers (ISPs) enterprises, building owners, cable companies, etc. Figure 1 indicates how LTE in the unlicensed spectrum could be deployed next to the current wireless infrastructure, where an LTE-U small cell could be either an LTE LAA small cell, controlled by a mobile operator, or a small cell operating solely in the unlicensed spectrum without mobile operator control. LTE is a technology that has been initially designed to operate in the licensed spectrum. Hence, it assumes that it can exclusively use the whole assigned spectrum, and therefore, it does not incorporate any techniques for harmonious coexistence with other possible co-located technologies.