Partial Contention-Free D2D Discovery for Proximity-Based Services in Cellular Networks

In this paper, we design a beacon-based Device-to-Device (D2D) neighbor discovery scheme as a key enabler for proximity-based services in the Long-Term Evolution-Advanced (LTE-A) system. The proposed scheme is based on the random access procedure of LTE-A system and it opts for contention-free access to enable the connection of D2D receivers while contention-based strategy is used to connect D2D transmitters. To allow neighbor discovery, D2D users first listen to peer uplink signals in order to detect beacons sent by D2D transmitters within their neighborhood. Then, the received beacons are forwarded by D2D receivers to the evolved Node B (eNB) to verify their unicity and to ensure the feasibility of establishing a D2D link, based on the location of involved users. At the successful D2D receiver side, new beacons will be assigned by the eNB while the initial beacons are kept to be used for D2D transmitters. This strategy allows to greatly reduce the collision probability for the whole network by avoiding collisions at the receiver side. It is worth noting that as the proposed scheme is based on almost the same principle of the random access procedure in the LTE-A system, it can readily be applied to the current architecture without significant modification. By means of simulations, we show that nearby user equipments can be found with low collision probability while achieving a high discovery rate proportional to the total number of users willing to initiate direct links with their neighbors.