The network and the other one is out of

The Device-to-device communication ca be classified
based on several aspects in different scenarios as follows

a)   
Coverage
– This aspect distinguishes whether the pair of D2D users is under the coverage
of cellular network. In this aspect the D2D communication can be categorized
as:

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·       
In
coverage –  Both D2D
users are within the coverage of the cellular network.

·       
Partial
coverage – One D2D user is in the coverage of the cellular
network and the other one is out of coverage.

·       
Out
of coverage – in this case both D2D users are outside
of the cellular communication network. Note that this scenario is mainly
considered in 3GPP for public safety cases, when the network can be temporarily
disabled.

b)   
Type of D2D communication
– this aspect classifies based on how many users are involved in D2D
communication:

·       
One
– to – one communication – direct communication between two
D2D users that creates one D2D communication pair.

·       
One
– to – many communication – in this one D2D user broadcasts or
multicasts 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 users
are served by the same cell or not:

·       
The
same cell – the D2D users form a D2D pair or a cluster are
attached to the same base station(eNB).

·       
The
different cell – The D2D users belonging to the same D2D
pair or cluster are attached to different base stations ( eNBs.)

d)   
Relaying functionality
– The D2D users can also have the relaying functionality to retransmit data of
other D2D users within its proximity. This feature can be mainly used to:

·       
Enhance
capacity – – The D2D user connected to another D2D user with
relay functionality is usually under the coverage of the eNB.

·       
 Extend coverage – The D2D user is in out
of coverage may use other D2D user as relay in order to reach the eNB.