# Energy-Efficient Device Discovery Mechanism for Device-to-Device Communications in 5G Networks +
## 1 Introduction  
The high transmission rate of fifth generation (5G) [1] networks has been strongly promoting the realization of Internet of Things (IoTs) and smart cities, while the big data resulting from IoTs or smart cities may overload the 5G network because of the limited spectrum resources. To resolve this problem, device-to-device (D2D) communications have emerged as an attractive choice recently. D2D communications allow devices to directly communicate without using the base station (BS) for relaying. Hence, D2D communications yield many merits [2, 3, 4] such as improving the spectral efficiency and system capacity, reducing network latency and energy consumption of UEs, offloading the 5G network, and extending the network coverage. The possible applications [3] of D2D consist of machine-to-machine (M2M), vehicle-to-vehicle (V2V) [4], content distribution [5], location-aware services in smart cities [6], social networking [7], public safety services [8], proximity online gaming, and e-health caring. However, to fulfill D2D communications, many challenging issues such as the device discovery, resource allocation, power control, interference management, and network security, remain to be solved [3, 9, 10, 11]. Among these issues, the first challenge of D2D communications is the device discovery. Therefore, lots of papers [12, 13, 14, 15, 16, 17, 18, 19, 20, 21] have studied anddesigned device discovery mechanisms for D2D communications. D2D discovery can adopt either core network functionalities to estimate proximity, also named the network-assisted D2D discovery [12; 13; 14; 15; 16; 17; 18], or autonomous actions taken by D2D-enabled devices [19; 20; 21]. [PERSON] et al. [12] designed a D2D discovery mechanism for proximity services using the random access procedure in the LTE-A system. The presented random-access-based D2D discovery mechanism adopts the network-assisted approach that enables the access or core network to centrally manage the formation of D2D communication networks. The paper [13] presented a signature-based discovery scheme as a conceptual example for cellular applications. [PERSON] et al. proposed two types of device discovery and access procedures in [14] and analyzed the performance of their proposed schemes using the Markov process model. The work [15] analyzed the performance of network-assisted D2D discovery in random spatial networks. The authors in [16] proposed a centralized D2D discovery mechanism using a signaling algorithm to exchange D2D discovery messages between devices. In this scheme, potential D2D pairs share uplink cellular spectrum resources with collision detection to initiate D2D links. In [17], a D2D discovery method, named ROOMMATEs, was proposed to minimize interference and energy consumption in the device discovery process. However, ROOMMATEs only uses Wi-Fi scans for peer discovery. D2D communications in [17] still use cellular bands. The authors in [18] proposed a neighborhood-aware method to increase the discovery probability by enabling neighborhood awareness. As to the paper [19], the authors proposed a social-aware peer discovery scheme, which exploits social network features for assisting the ad hoc peer discovery for D2D communications. In this scheme, D2D users are first categorized into groups according to their social metrics of communities and centralities. Subsequently, the optimal beaconing rate is determined for each group of users. Since many proximity devices may send discovery beacons at the same time, collisions may occur, leading to device discovery failures. Therefore, to increase the discovery probability and reduce the discovery delay, [PERSON] et al. [20] introduced a random backoff procedure to retransmit beacons. In the paper [21], [PERSON] et al. proposed a simple and efficient mechanism, called listen channel randomization (LCR), to speed up device discovery in Wi-Fi Direct. In the proposed method of [21], a Wi-Fi Direct device selects a less interfered social channel when it attempts to wait for other devices' probe requests.