# User-Cell Association for Security and Energy Efficiency in Ultra-Dense Heterogeneous Networks
## 1 Introduction
### State-of-the-Art  
In PLS, a non-zero secure rate can be obtained by any strategy that produces an advantage in terms of signal-to-noise ratio (SNR) over the attacker. This opened the research to study different strategies, from transmit power optimization to artificial noise injection.  
The joint optimization of the secrecy rate and the total consumed power was studied in [13]. Therein, the instantaneous received SNR for the legitimate terminal is assumed to be strictly larger than the eavesdropper receiver. The extension to multi-antenna scheme (MIMO) is investigated in [14], considering the downlink. In [15], a power allocation policy was developed to maximize the secrecy information rate while maintaining the harvested energy requirement of the energy receiver.  
The joint optimization of secrecy and energy efficiency has been recently investigated in Energy Harvesting scenarios. In [16; 17], the maximization of the secrecy energy efficiency (SEE) is obtained by means of PLS-based signal processing. In [12], a multi-objective optimization problem is targeted to full-duplex (FD) networks with simultaneous wireless information and power transfer (SWIPT).  
In [18], an energy-efficient relay selection scheme which jointly considers best relay selection and dynamic power allocation in order to maximize the secrecy rate as well as to minimize energy consumption is provided. The role of the interference in providing PLS in (downlink only) internet of things (IoT) is considered in [19], where also the cost in terms of energy is calculated. In [20], a radio resource allocation framework to optimize both the confidentiality and the energy efficiency of a communication system is proposed. In [21], cross-layer cooperation as a viable solution for the achievement of reliability and energy efficiency in wireless communication is proposed. A survey on energy efficient design of wireless networks can be found in [22].  
For what concerns the cell-user association, in the traditional approach the user terminal (UE) selects the base station (BS) that provides the highest signal-to-noise plus interference ratio (SINR). Authors in [23; 24] propose optimization models for wireless network design under SIR and users-to-BS association constraints. Other approaches involve a meta-heuristic approach for a robust association that also takes into account the uncertainty of data rate transmissions [25]. Application to sensors can be found in [26], where an optimal user association strategy for large-scale Internet-of-Things (IoT) sensor networks is illustrated.  
Only a few papers consider network security. In [27], the UE selects the BS that provides the highest secrecy rate, but inter-cell interference is not considered, while it is a basic element of UDNs. Differently in [28], in-band interference is considered and the maximum secrecy rate association is approximated using the Maclaurin formulation. In [29], the maximum secure area is considered. There are also papers that consider energy efficiency as association policy as in [30; 31; 32; 33]. In [34], the transmission power is minimized by minimizing the number of active small-cells with a constraint on the minimum achievable rate. The complexity of the problem and a heuristic solution are evaluated. Finally, in [35; 36] the relations between secrecy capacity and energy consumption are investigated in heterogeneous cellular networks and original metrics that bind the secure area of a cell, the afforded date-rate and the power spent by the BS are proposed as a tool for the evaluation of different joint optimization strategies.