  Device-to-device (D2D) communication will likely be added to LTE in 3GPP
Release 12. In principle, exploiting direct communication between nearby mobile
devices will improve spectrum utilization, overall throughput, and energy
consumption, while enabling new peer-to-peer and location-based applications
and services. D2D-enabled LTE devices can also become competitive for fallback
public safety networks, that must function when cellular networks are not
available, or fail. Introducing D2D poses many challenges and risks to the
long-standing cellular architecture, which is centered around the base station.
We provide an overview on D2D standardization activities in 3GPP, identify
outstanding technical challenges, draw lessons from initial evaluation studies,
and summarize "best practices" in the design of a D2D-enabled air interface for
LTE-based cellular networks.
