  In this paper, we address the problem of optimal relay placement in a
cellular network assuming network densification, with the aim of maximizing
cell capacity. In our model, a fraction of radio resources is dedicated to the
base-station (BS)/relay nodes (RN) communication. In the remaining resources,
BS and RN transmit simultaneously to users. During this phase, the network is
densified in the sense that the transmitters density and so network capacity
are increased. Intra- and inter-cell interference is taken into account in
Signal to Interference plus Noise Ratio (SINR) simple formulas derived from a
fluid model for heterogeneous network. Optimization can then be quickly
performed using Simulated Annealing. Performance results show that cell
capacity is boosted thanks to densification despite a degradation of the signal
quality. Bounds are also provided on the fraction of resources dedicated to the
BS-RN link.
