# Energy-Efficient Device Discovery Mechanism for Device-to-Device Communications in 5G Networks +
## 1 Introduction
According to the experimental results, LCR can reduce the discovery delay up to 72%, compared with the legacy Wi-Fi Direct. There exist two types of D2D discovery approaches: direct and network-assisted. In direct discovery schemes, D2D devices autonomously and distributedly discover, or show their presence to, other devices in proximity. Frequently used protocols for direct and distributed discovery purposes include Wi-Fi Direct [21; 22] and Bluetooth [23]. In network-assisted discovery mechanisms, the evolved packet core (EPC) and BS use the location information of UEs to trigger the D2D discovery process that can be either centralized or distributed. The network-assisted D2D discovery can reduce the energy consumption, signaling overhead, and interference in D2D discovery, by exploiting knowledge of the network layout. However, the network-assisted approach does not work if the devices are out of network coverage. On the other hand, the majority of literatures designed D2D discovery schemes for proximity services [12; 13; 17; 19; 20]. Very few papers have proposed D2D discovery mechanisms for finding a specific target device [15]. Additionally, most of the D2D discovery mechanisms in the literature are either fully centralized or fully distributed and perhaps not optimal. To summarize, the direct discovery method (e.g., Wi-Fi Direct-based) can use unlicensed bands for discovering UEs and thus save cellular spectrum resources. However, the direct discovery scheme may need to retransmit messages several times because it uses the random access protocol and thus contentions and collisions are inevitable. Hence, the direct discovery scheme is highly energy and time-consuming. To reduce the energy consumption of UEs, it is necessary to reduce contentions or collisions in the direct discovery scheme. One effective way to achieve this goal is to sift D2D candidates from all UEs using the network-assisted approach, in which the BS or EPC can use UEs' location information to judge whether two UEs are close enough to initiate D2D. If two UEs are too far away from each other, the BS or EPC notifies them of directly using cellular services for communications, thus avoiding unnecessary D2D discovery overhead and reducing collisions of D2D discovery messages and UEs' power consumption. Consequently, in [24] we designed a hybrid mechanism combining the network-assisted and direct D2D discovery approaches to find a specific target device in proximity and evaluated its performance using simulations. In this extended paper, we further modify our previous design [24] in several aspects to improve the performance of the proposed hybrid D2D discovery mechanism. More specifically, two additional novel algorithms, channel assignment algorithm and binary exponential backoff without frozen backoff time algorithm, are proposed in this paper to further reduce the discovery delay. Additionally, to objectively and scientifically evaluate the performance of the proposed mechanism, in this paper we theoretically analyze the energy consumption, discovery success rate, and discovery delay of the proposed mechanism. Finally, several examples are presented to show the superiority of the proposed mechanism over other existing discovery schemes and validate the accuracy of our theoretic analyses. The rest of this paper is organized as follows. Section 2 addresses related works about D2D discovery schemes. Section 3 introduces the proposed hybrid D2D discovery mechanism that combines the network-assisted and direct discovery techniques. Section 4 analyzes the performance of the proposed D2D discovery mechanism. Section 5 presents evaluation results to demonstrate the superiority of the proposed hybrid D2D discovery mechanism. Finally, the concluding remarks are made in Section 6.