Towards Energy-Efficient D2D Communications With Multiple Transmission Modes:A Resource Joint Allocation Approach

The Device-to-Device(D2D)communication supports two users in proximity to directly communicate with each other reusing cellular links when the network is overload.Employing an effective resource allocation strategy to mitigate the interference,the spectrum utilization and the system capacity can be jointly improved.Besides,the introduction of the relay technology can realize multiple transmission modes,such as direct mode,twohop mode and cooperative mode,and extend communication coverage,further enhancing D2 D performance.However,two-hop mode and cooperative mode bring additional power consumption and make the interference situation more complicated.In order to highlight superiority of D2 D communications,and meet the needs of high rate and high energy efficiency(EE)in 5G,this paper studies the three-dimensional(3-D)resource joint management approach to optimize EE for D2 D communications with multiple transmission modes.The main contributions are summarized as follows:From the perspective of the system EE,considering direct mode,two-hop mode,and cooperation mode,we focus the 3-D joint problem on power control,mode selection and channel allocation.By employing the orthogonality of D2 D links and the uniqueness of transmission mode,the original problem is decoupled,and an optimal scheme is proposed to maximize the system EE.Besides,a low-complexity sub-optimal is proposed to reduce the running time and approximate the performance of the optimal solution.The simulation results show that compared with the two-dimensional(2-D)resource joint management approach,which measures only direct mode and two-hop mode,the 3-D resource management scheme proposed in this paper has obvious promotion in the system EE,and the EE by the sub-optimal scheme also reaches about 96.2% of the EE by the optimal scheme on average.Some D2 D pairs have lower EE when the optimal target is system EE.In this paper,we further study the fairness-based resource management approaches.On one hand,considering the optimal fairness,we proposed a 3-D resource allocation scheme to maximize the minimum EE.The simulation results show that the resource allocation scheme with three transmission modes can enhance the fairness.On the other hand,introducing the fairness constraint,using the system EE as the optimal target and the minimum EE as the constraint,we proposed a power-mode-channel resource joint allocation approach to maximize the min-imum EE.The adjustment of the minimum EE threshold within a certain range will achieve a compromise between the optimal system EE and the optimal fairness for the D2 D system.