IEEE Communications Magazine • July 2015 26 0163-6804/15/$25.00 © 2015 IEEE Konstantinos Chatzikoko- lakis and Nancy Alonisti- oti are with National and Kapodistrian University of Athens. Panagiotis Spapis and Alexandros Kaloxylos are with Huawei Technologies. INTRODUCTION Traffic analysis indicates that mobile and wire- less networks will have to cope with a huge increase of data traffic over the next decade. This will occur due to the vast proliferation of mobile and wireless devices and the growing need for traffic volume per subscriber. More specifically, over 4 billion mobile devices (i.e., laptops, tablets, and smartphones) exist in the mobile service market, and an 11-fold increase of mobile data traffic by 2018 compared to 2013 is expected [1]. In addition, the actual traffic vol- ume per subscriber increases 25–40 percent per year exceeding; hence, the expectations set by the International Telecommunication Union (ITU) [2, 3]. The Third Generation Partnership Project (3GPP), motivated by the increased mobile data traffic volume, has encouraged the research community to move in three directions: • Spectral efficiency improvement • Higher network cell density • Exploitation of underutilized radio spec- trum resources [4] The first solution includes coordinated multi- point (CoMP) transmission using sophisticated multiple-input multiple-output (MIMO) tech- niques and interference management mecha- nisms. The second area deals with the addition of extra layer cells in the network with base sta- tions (BSs) that cover smaller areas compared to macro and micro BSs. These solutions include femtocells and the use of relay nodes. In this article, we focus on the third aspect, which deals with the extension of spectrum oppor- tunities for mobile broadband access. Nowadays, a mobile network operator (MNO) acquires from the national regulatory authority (NRA) spectrum resources, which through network planning are allocated in different geographical areas. The re- allocation of underutilized spectrum resources to congested areas is a slow process that requires thorough investigation of underused or overload- ed frequency bands (for months or even years) and may require spectrum refarming. However, in the near future spectrum scarcity will call for more dynamic, flexible, and fast solutions. Up to now, flexible radio spectrum management has not been a preferable option because the allocated spectrum is more or less sufficient to cover the current needs of MNOs. Moreover, MNOs are reluctant to share or acquire spectrum to/from other operators or licensed spectrum users. In the near future, spectrum sharing will be a necessity since the aforementioned alternative solutions (e.g., MIMO, CoMP) will not be able to cover MNOs’ needs in capacity. Thus, new schemes to identify spectrum opportunities and decide the most profitable choice for MNOs have to be designed. Toward efficient spectrum management, sev- eral authorization regimes and spectrum access schemes are currently used or being investigated. These regimes may be divided into two cate- gories based on the license that is provided to the users: individual authorization approaches and general authorization approaches (also known as license exempt or unlicensed). In individual authorization regimes we distin- guish exclusive, co-primary and licensed shared access (LSA) schemes. In general authorization regime, unlicensed shared access (i.e. Wi-Fi, Bluetooth, etc.), and secondary horizontal shared access (i.e. TV White Spaces) are used, but do not manage to fill the capacity gap due to lack of quality of service (QoS) guarantees for end users ABSTRACT The vast increase in the number of mobile devices and their mobile traffic demands indi- cates the need for additional spectrum for cellu- lar communications. Since it is not trivial to allocate exclusively new spectrum bands for cel- lular communications, it is imperative to improve the spectrum usage through new spectrum shar- ing mechanisms. This implies that the mobile network operators will have to cooperate and interact to cover the augmented traffic require- ments. In this article we present a novel archi- tectural framework that enables the mobile network operators and other spectrum license holders to exchange information about spectrum availability. We also present a novel spectrum sharing mechanism based on fuzzy logic to facili- tate operators in selecting the most suitable spectrum to cover their needs. 5G SPECTRUM: ENABLING THE FUTURE MOBILE LANDSCAPE Konstantinos Chatzikokolakis, Panagiotis Spapis, Alexandros Kaloxylos, and Nancy Alonistioti Toward Spectrum Sharing: Opportunities and Technical Enablers