The Device-to-device communication ca be classifiedbased on several aspects in different scenarios as followsa) Coverage– This aspect distinguishes whether the pair of D2D users is under the coverageof cellular network. In this aspect the D2D communication can be categorizedas:· Incoverage – Both D2Dusers are within the coverage of the cellular network.· Partialcoverage – One D2D user is in the coverage of the cellularnetwork and the other one is out of coverage.· Outof coverage – in this case both D2D users are outsideof the cellular communication network. Note that this scenario is mainlyconsidered in 3GPP for public safety cases, when the network can be temporarilydisabled.b) Type of D2D communication– this aspect classifies based on how many users are involved in D2Dcommunication:· One– to – one communication – direct communication between twoD2D users that creates one D2D communication pair.
· One– to – many communication – in this one D2D user broadcasts ormulticasts data to several D2D users in a cluster. This is also called device –to multi – device (D2MD).c) Area of D2D communication– this aspect mainly distinguishing whether both the communicating D2D usersare served by the same cell or not:· Thesame cell – the D2D users form a D2D pair or a cluster areattached to the same base station(eNB).
· Thedifferent cell – The D2D users belonging to the same D2Dpair or cluster are attached to different base stations ( eNBs.)d) Relaying functionality- The D2D users can also have the relaying functionality to retransmit data ofother D2D users within its proximity. This feature can be mainly used to:· Enhancecapacity – – The D2D user connected to another D2D user withrelay functionality is usually under the coverage of the eNB.· Extend coverage – The D2D user is in outof coverage may use other D2D user as relay in order to reach the eNB.