# User-Cell Association for Security and Energy Efficiency in Ultra-Dense Heterogeneous Networks
## 1 Introduction  
The last decades have been characterized by an unprecedented fast development of wireless technologies. The 5 th generation (5G) of mobile network is nowadays being deployed and 6G is under investigation. Pervasive wireless connection is mandatory to meet the increasing demand of digital services, during the everyday life of citizens [1]. In order to meet the goal of a pervasive connectivity and to provide services with different constraints (from ultra low latency to ultra high speed), cells of different size and with different features should coexist and collaborate: the paradigm of ultra dense network (UDN) [2]. UDN is considered one of the best ways to meet user expectations and support future wireless services development [3]. The more the services on-demand and on-mobility, anytime-anywhere, the more the amount of information in the air, often critical from the point of view of privacy, but not only. The data which every minute flows in the wireless channel can be very critical, from body/health information to vehicles assisted/autonomous driving or industrial automation. The word \"smart\" is nowadays pre-imposed to several key-words: factory, farm, vehicle, body, etc. One of the main assumptions under this word is the always-on wireless connectivity, since smartness implies flow of information, often in real-time. Unfortunately, the benefit of wireless connectivity is also an issue: the channel can be heard by any receiver, legitimate or not. In order to trust the digital services of the future, security must be inserted as a key feature, from the very beginning of the design of the wireless system/technology [4]. physical-layer security (PLS) is envisioned as a promising technique to provide an additional level of security, since it does not rely on the assumptionof limited computational power of the attacker, as the classical cryptography does. While in cryptography, a message is correctly taken by the attacker from the wireless channel but its meaning cannot be interpreted, in PLS the attacker is not able to correctly detect any message by analyzing the channel [5]. There is another critical issue that all approaching technologies should seriously take into account: the energy consumption. The Information and Communication Technology (ICT) sector is one of the main contributors to the increase of CO\({}_{2}\) level on Earth [6]. A conservative estimation currently puts around 4% of all electricity consumption and over 2% of all CO\({}_{2}\) emissions as the result of ICT use. If 6G-like on-demand on-mobility services are added, the consumption numbers are envisioned to double [7]. It is extremely important for the success of both 5G and 6G services to intrinsically provide minimum consumption, i.e., to meet the energy efficiency by design. 6G is the first mobile technology which is evaluating to directly impose energy efficiency constraints (Terabyte/Joule) to move information. Energy efficiency and security are, thus, two of the key issues that next generation wireless connectivity has to face. This is particularly true for UDNs. Indeed UDNs offer new opportunities for achieving PLS because wireless channels are more random and inter-cell interference can be beneficial. Using suitable interference management systems, the legitimate user can benefit form network densification while the eavesdropper experiences strong interference that degrades its signal quality [8]. Moreover, cells' densification allows to provide coverage with low-power access points; thus, if the cells are suitably activated depending the traffic on a given area, UDNs can be a mean to improve energy efficiency [9]. It is important to point out that the upcoming 5G and the future 6G communications will have to cope with an extremely large set of objectives and figures of merit, such as enhanced throughput, very short latency, a generalized coverage with stable user experience for any possible speed of the devices, unprecedented spectral and energy efficiencies, and so on.