Direct (or D2D) communications allow two UEs to communicate without passing through the eNodeB. However, the two UEs may still need to relay their communication through the eNB from time to time, hence should be able to switch from the direct to the relayed mode seamlessly, without this affecting the QoS. In this paper we show that in conventional systems a mode switching may cause relevant losses, and propose two architec- tures to mitigate or solve this problem. Our proposals do not require extra signaling or additional functionalities to be added to the network, hence are scalable and inexpensive. We assess their effectiveness through detailed system-level simulations.

Keywords: {LTE-A, device-to-device, mode switching}