  Device-to-device (D2D) communications in cellular networks are promising
technologies for improving network throughput, spectrum efficiency, and
transmission delay. In this paper, we first introduce the concept of guard
distance to explore a proper system model for enabling multiple concurrent D2D
pairs in the same cell. Considering the Signal to Interference Ratio (SIR)
requirements for both macro-cell and D2D communications, a geometrical method
is proposed to obtain the guard distances from a D2D user equipment (DUE) to
the base station (BS), to the transmitting cellular user equipment (CUE), and
to other communicating D2D pairs, respectively, when the uplink resource is
reused. By utilizing the guard distances, we then derive the bounds of the
maximum throughput improvement provided by D2D communications in a cell.
Extensive simulations are conducted to demonstrate the impact of different
parameters on the optimal maximum throughput. We believe that the obtained
results can provide useful guidelines for the deployment of future cellular
networks with underlaying D2D communications.
