In recent, amount of traffic to be treated by cellular network increases as mobile multimedia services become popular. Especially, evolved nodeB (eNB) handles traffic more than in past years because of fast-growing needs of high data rate services. In pre-existing cellular networks, eNB should relay UEs' (User Equipment) data to core network even though users located in same cell coverage communicate with each other, which increases communication delay and burdens eNBs because of densely crowded users.
To solve this problem, device-to-device (D2D) communication has been introduced, which enables UEs to either reuse the frequency band used in pre-existing cellular network or use out-of-band frequency for direct communication with each other. D2D communication can reduce end-to-end delay and traffic load on eNBs.
FIG. 1 shows the wireless communication system 100 in which the D2D technology implements. As shown, the wireless communication system 100 includes an eNB 110 serving three UEs 114A-C and a core network 120 comprising conventional network nodes, such as MME (Mobility Management Entity) 112, S-GW (Serving Gateway) 116, P-GW (PDN Gateway) 118, HSS, etc.
It should be noted that, although FIG. 1 depicts only one eNB and three UES, it is only for the purpose of illustration and the wireless communication system 100 can accommodate any number of eNB(s) and UE(s).
In an exemplary scenario, UEs 114B and 114C which are being in communication via an infrastructure communication path 122A-C are being moved to be in close proximity to each other. Then, for example for the sake of power saving, cost saving, and/or offload of the core network etc., it may be necessary to switch the infrastructure communication path between them to a D2D communication path, for transporting traffic directly between the two UEs. In another exemplary scenario, two UEs 114A and 114B which are being in communication via a D2D communication path 124 are moved away from each other. Then, transport conditions of the D2D link 124 may become bad, so that the D2D communication path is not available between the two UEs. As such, to avoid communications between the two UEs being interrupted, the two UEs may have to switch the communications from the D2D communication path to other available communication path, such as an infrastructure communication path.
For network controlled or assisted D2D discovery and communication, at least one of the UEs involved in D2D may need to be kept in RRC_CONNECTED state for network control and mobility management. For autonomous D2D, the devices thereof may be in either RRC_IDLE or RRC_CONNECTED state.
However, since D2D may be deployed in a band separated from that of the controlling cell or serving network and it is important to keep control overhead as low as possible and utilize offloading possibility as much as possible for the serving network, there is a need to improve the current D2D discovery and communication procedure.