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4 Current Progress in Standardization
As early as the 3GPP meeting held in June 2011, a study item description on the
radio aspects of D2D discovery and communication was submitted by Qualcomm.
Meanwhile, a study item description on LTE Direct (LTE-D) was submitted to the
3GPP meeting held in August 2011, which proposed the study of the service
requirement for direct over-the-air LTE D2D discovery and communication. The
meeting of November 2011 studied use cases and identified potential requirements
for a network operator controlled discovery and communication between proxi-
mate devices. As the finalization of LTE Rel-11, 3GPP initiated the new step on
Rel-12 and beyond which started as a workshop in June 2012. At that workshop, it
was agreed that the machine type and short-range communication scenarios should
be embraced to give rise to new traffic types. As a follow-up to the workshop, it
was agreed to start a study on LTE Proximity Services (ProSe) in the Radio Access
Network (RAN) 58 plenary meeting in December 2012 [4]. The study on ProSe
includes two parts, namely D2D discovery and D2D communication. The main
results on D2D use cases and potential requirements are captured in [5] and the
needed architectural enhancements to support ProSe in [6]. The current work on
LTE D2D device discovery and D2D communication mainly focuses on the
technical details, including discovery signal design, resource allocation and
scheduling, synchronization mechanism, etc. For IEEE to continue Wi-Fi Direct
research, IEEE 802.11 Infrastructure Service Discovery Study Group has done
much work on proximate discovery and communication with low energy, long
range (up to 500 m), and large scale (up to 1,000 mobile devices) for mobile social
networks since 2010.
5 D2D Functional Block in LTE-A SAE Architecture
3GPP technology has the opportunity to become the platform of choice to enable
D2D communication. The ProSe can be divided into two parts, proximity dis-
covery and direct communication, as shown in Fig. 2a.
With proximity discovery, users can discover other users that are in the prox-
imity. Discovery mechanisms can be network or user-assisted. Proximity discov-
ery can be a standalone service to users, for example to enhance social networking
services and does not need to trigger direct communication. Direct communication
refers to devices communicating directly, without going via a base station, when
they are within reach of each other. Users may initiate direct communication
directly without proximity discovery. However, proximity discovery is a natural
trigger for direct communication that can reduce the need for manual interaction.
In particular, when direct communication is integrated with a cellular network, it is
considered natural that it is preceded by network supported proximity discovery.
From an architectural view the solution for ProSe involves both UEs, RAN, core
Introduction to D2D Communication 7