  Device-to-device (D2D) communication underlaying cellular wireless networks
is a promising concept to improve user experience and resource utilization by
allowing direct transmission between two cellular devices. In this paper,
performance of network-assisted D2D communication is investigated where D2D
traffic is carried through relay nodes. Considering a multi-user and
multi-relay network, we propose a distributed solution for resource allocation
with a view to maximizing network sum-rate. An optimization problem is
formulated for radio resource allocation at the relays. The objective is to
maximize end-to-end rate as well as satisfy the data rate requirements for
cellular and D2D user equipments under total power constraint. Due to
intractability of the resource allocation problem, we propose a solution
approach using message passing technique where each user equipment sends and
receives information messages to/from the relay node in an iterative manner
with the goal of achieving an optimal allocation. Therefore, the computational
effort is distributed among all the user equipments and the corresponding relay
node. The convergence and optimality of the proposed scheme are proved and a
possible distributed implementation of the scheme in practical LTE-Advanced
networks is outlined. The numerical results show that there is a distance
threshold beyond which relay-aided D2D communication significantly improves
network performance with a small increase in end-to-end delay when compared to
direct communication between D2D peers.
