Outband D2D:
Here the D2D links exploit unlicensed
spectrum. The motivation behind using outband D2D commu-
nication is to eliminate the interference issue between D2D
and cellular link. Using unlicensed spectrum requires an extra
interface and usually adopts other wireless technologies such
as WiFi Direct [30], ZigBee [40] or bluetooth [31]. Some of
the work on outband D2D (see e.g., [12], [13], [32], [33])
suggest to give the control of the second interface/technology
to the cellular network (i.e., controlled). In contrast, others
(see e.g., [35]) propose to keep cellular communications
controlled and leave the D2D communications to the users
(i.e., autonomous). Outband D2D uses unlicensed spectrum
which makes the interference issue between D2D and cellular
users irrelevant. On the other hand, outband D2D may suffer
from the uncontrolled nature of unlicensed spectrum. It should
be noted that only cellular devices with two wireless interfaces
(e.g., LTE and WiFi) can use outband D2D, and thus users can
have simultaneous D2D and cellular communications.
Fig. 3 illustrates the taxonomy introduced for D2D communications in cellular networks. In the following sections,we review the related literature based on this taxonomy.