  Device-to-Device (D2D) communications is a technology that allows mobile
users to relay information to each other, without access to the cellular
network. In this paper, we consider how to dynamically select multi-hop routes
for D2D communications in spectrum co-existence with a fully loaded cellular
network. The modelling scenario is that of a real urban environment, when the
cellular network is congested during an unexpected event, such as a terrorist
attack. We use D2D to relay data across the urban terrain, in the presence of
conventional cellular (CC) communications.
  We consider different wireless routing algorithms, namely:
shortest-path-routing (SPR), interference-aware-routing (IAR), and
broadcast-routing (BR). In general, there is a fundamental trade-off between
D2D and CC outage performances, due to their mutual interference relationship.
For different CC outage constraints and D2D end-to-end distances, the paper
recommends different D2D routing strategies. The paper also considers the
effects of varying user density and urban building material properties on
overall D2D relaying feasibility. Over a distance of a kilometre, it was found
that the success probability of D2D communications can reach 91% for a moderate
participating user density (400 per square km) and a low wall penetration loss
(<10dB).
