  This dissertation studies different aspects of D2D communications and its
impact on the key performance indicators of the network. We design an
architecture for the collaboration of cellular users by means of timely
exploited D2D opportunities. We begin by presenting the analytical study on
opportunistic outband D2D communications. The study reveals the great potential
of opportunistic outband D2D communications for enhancing energy efficiency,
fairness, and capacity of cellular networks when groups of D2D users can be
form and managed in the cellular network. Then we introduce a protocol that is
compatible with the latest release of IEEE and 3GPP standards and allows for
implementation of our proposal in a today's cellular network. To validate our
analytical findings, we use our experimental Software Defined Radio (SDR)-based
testbed to further study our proposal in a real world scenario. The
experimental results confirm the outstanding potential of opportunistic outband
D2D communications. Finally, we investigate the performance merits and
disadvantages of different D2D modes. Our investigation reveals, despite the
common belief, that all D2D modes are complementary and their merits are
scenario based.
