Energy efficiency optimization based on interference alignment for device-to-device MIMO downlink underlaying cellular network

This paper intends to adopt interference alignment (IA) for improving the energy efficiency (EE) of both the device-to-device (D2D) and cellular links in a D2D multiple-input and multiple-output (MIMO) downlink underlaying cellular network. Due to the strong cross-tier interference, the EE optimization problem becomes NP-hard and computationally difficult to solve. IA is implemented firstly to mitigate the cross-tier interference and decouple the problem into independent EE optimization for D2D and cellular links, and then an energy-efficient MIMO transmission scheme which further enhances the EE is proposed. Furthermore, via the nonlinear fractional programming and modified water-filling method, the closed-form expression of the achieved maximal EE for D2D link is derived while satisfying the IA conditions. Simulation results confirm that the EE of the proposed scheme approaches a peak value when both the maximum transmit power allowances of D2D and cellular links are large enough. Meanwhile, the system energy and spectrum efficiency can be improved significantly compared with the conventional interference cancelation in D2D underlay network.