D2D mobility management

Signal quality measurement: 

UEs involved in D2D communication measure sidelink quality. In 5G, these measurements can be performed when needed and requested

Coordination between the source and target BSs: 

The 5G RAN may also communicate with BSs of other AIVs, within the same Public Land Mobile Network (PLMN) or beyond. Enhancements to the X2 interface (or Xn interface in NR terminology) are required to support this in order to achieve inter‐RAT D2D compatibility and service continuity

Resource allocation by the target BS: 

After the source 5G RAN decides upon the target 5G RAN to which D2D UEs should be handed over, the make‐before‐break rationale needs to be adopted. This rationale requires that the D2D context information is transferred from the source 5G RAN to the target 5G RAN before the actual handover execution. Context information here refers to various aspects including D2D-specific information such as communication types, radio resources, traffic type, D2D group, etc. This enables the target 5G RAN to prepare customized D2D resources for the D2D unicast or group‐cast communication. The handling of D2D resources across different AIVs is another issue. One approach may be that all D2D UEs use the original D2D resources for D2D communications until all D2D UEs hand over to the new target RAN/AIVs. 

Packet switch from the source BS to the target BS: 

After the handover decision is completed and a handover command is sent to the D2D UEs, they execute the handover and might use original, temporary or newly assigned D2D resources in order to continue the ongoing D2D

communications.