# Energy-Efficient Device Discovery Mechanism for Device-to-Device Communications in 5G Networks +
## 2 Related Works  
In addition to the cellular network technology, other existing techniques [13] such as Infrared Data Association (IrDA), Wi-Fi Direct [21; 22], and Bluetooth [23], are frequently used for D2D discovery. IrDA uses a slotted-ALOHA structure to resolve the multiple access problem in device discovery. Bluetooth uses 32 channels out of the available 79 Bluetooth channels for device discovery. A discovering device sends an inquiry signal on one of the 32 discovery channels for a half slot and switches to a new channel to send in the second half. The discovering device repeats the procedure by cycling through the 32 discovery channels until the discovery period expires. A device to be discovered periodically executes the discovery scan procedure by listening for the discovery signal on one of the 32 channels. Wi-Fi Direct defines two states, i.e., search and listen, in the find phase, and uses carrier sense multiple access with collision avoidance (CSMA/CA) to discover devices [21; 22]. In the search state, a device sends probe request messages on one of the Wi-Fi channels (e.g., channels 1, 6, and 11) using the CSMA/CA and waits for the probe response until timeout, as shown in Figure 1. This searching process is repeated on each of the three channels 1, 6 and 11. A device in the listen state selects one of the channels to listen for probe request messages until the period of the listen state expires. In the find phase, the state of a device continuously alternates between search and listen until a device is discovered or timeout occurs. A brief comparison between Bluetooth 5.0 and Wi-Fi Direct is given in Table 1. However, these two protocols use asynchronous transmission technique and are not efficient in terms of discovery delay and discovery success rate. Thus, the FlashLinQ protocol [25] which adopts synchronous transmission was proposed to improve the discovery performance. It is important to minimize the collision probability in contention-based D2D discovery mechanisms. To optimize the probability of successful transmissions of discovery signals, in [16] the transmission probability at each time slot is set to \(1/N\) if there are \(N\) D2D pairs. To reduce the collision probability occurring in the device discovery phase, a random backoff algorithm was proposed for D2D discovery in [20]. [PERSON] et al. [26] proposed a theoretical model to calculate the optimal transmission probability based on a given set of parameters such as the number of UEs and resources. However, [26] assumes a priori knowledge of the number of UEs in the discovery group, which is usually not the case in reality. Additionally, it considers an ideal propagation model, disregarding fading and interference factors. To improve the discovery performance, the paper [27] proposed an adaptive algorithm which considers the available resources and the number of nearby UEs as they are being discovered, and adapts the transmission probability which is computed using the formula given by [26] and rounds to the nearest multiple of 0.25. To improve the discovery performance of contention-based schemes, [PERSON] et al. proposed a non-orthogonal spectrum-sharing scheme, which is based on trellis tone modulation multiple-access (TTMMA), for D2D discovery [28]. TTMMA allows several UEs to modulate and transmit their signals on the same resource segment. This design mitigates the need for strict collision avoidance and increases the discovery capacity. In [29], an overlapping community-aware neighbor discovery mechanism for D2D communications was proposed. By dynamically estimating the roles of overlapping community UEs, the beacon detection rates can be derived. Additionally, based on the connection status of other intra-community and inter-community UEs, the beacon detection rates can be dynamically adjusted to improve the energy efficiency and discovery rates.