Multiple device-to-device users underlaying downlink cellular networks with superposition coding

Device-to-Device (D2D) communications has been proposed to provide high data rate service via direct transmissions between devices. Cooperation between the cellular user (CU) and the D2D user can be achieved using superposition coding, where the D2D transmitter (DT) allocates some of its transmission power to forward the CU’s traffic, and transmits to its own D2D receiver (DR) with the remaining power. The sum rate of the cellular and D2D networks in existing schemes are limited by allowing only a single pair of D2D users underlaying the cellular network. In this paper, we propose several new spectrum access policies involving multiple D2D pairs. Despite the additional interference, it is shown that both the CU and the individual D2D users’ achievable rates can be improved simultaneously by adjusting the power allocation factor in the superposition coding, resulting in a significant gain in the sum rate when the number of D2D pairs are below a threshold. This threshold is quantified using second order approximations for the average achievable rates for both the CU and the individual D2D users for different policies.