| As communications technology continues to evolve and mobile devices explode,the requirements are increasing for coverage in order to guarantee the user experience,which can be achieved by not only improving base stations to expand coverage,but also by relaying more mobile terminals.In addition to the user experience,energy efficiency and spectrum efficiency must also be taken into account,which makes the optimisation of energy and spectrum efficiency in relay networks particularly important.In this paper,we investigate the energy and spectrum efficiency of wireless multi-relay communication networks.The three main scenarios,namely,amplify-and-forward multi-relay-assisted unidirectional communication,amplify-and-forward multi-relay-assisted bidirectional communication and amplify-and-forward multi-relay-assisted D2 D communication,are investigated in turn,and corresponding optimisation solutions are presented.The main work and contributions of this paper are as follows:1.A relay selection and energy efficiency optimisation method is proposed for amplify-and-forward multi-relay assisted unidirectional communication networks.The method takes the condition of non-perfect channels as a premise,considers the case of multiple relays,takes energy efficiency maximisation as the ultimate optimisation objective,and divides the energy efficiency maximisation problem into two sub-problems to solve: the first sub-problem is relay selection,and the second sub-problem is spectrum efficiency optimisation.For the first sub-problem,a relay selection algorithm is designed for determining the location of the optimal relay by expressing the power consumption of the source and relay nodes as an expression related to the location of the relay,with the objective of amplifying the total power consumption of the forwarding multi-relay-assisted unidirectional communication network.For the second sub-problem,after the location of the optimal relay is determined,the energy efficiency maximisation problem is eventually turned into a 0-1 planning problem related to the spectral efficiency,as can be seen from the existing energy efficiency expression,and then the algorithm for solving the optimal spectral efficiency is designed according to the Dinkelbach algorithm.Simulation results show that the proposed optimisation method can achieve higher energy efficiency spectral efficiency with less power consumption than methods such as optimising only the relay position or optimising only the power.2.A relay selection and energy-efficiency optimization method is proposed for amplify-and-forward multi-relay-assisted two-way communication systems.The method has a symmetric channel for two-way communication,and inherits the previous one-way non-perfect channel premise and energy-efficiency maximization objective,and transforms the energy-efficiency maximization optimization problem into two sub-optimization problems of relay selection and spectral efficiency optimization.Here,the former suboptimization problem is to turn the power consumption of the amplify-and-forward multi-relay-assisted two-way communication system into the power consumption of three nodes,i.e.,the source node,the relay node and the destination node;by expressing the power consumption problem of each node as an expression related to the relay position,the optimal relay is finally selected with the goal of minimizing power consumption.In the latter sub-optimization problem,when the optimal relay position is obtained,the energy efficiency becomes an expression related to the spectral efficiency only,and the solution of the optimal spectral efficiency that maximizes the energy efficiency is solved by derivation.Simulation results show that the proposed optimization method can achieve higher energy-efficient spectral efficiency in symmetric channels with less power consumption than the methods of only optimizing the relay location or only optimizing the power.3.For the scenario of amplify-and-forward multiple relay-assisted device-to-device(D2D)communication,an optimisation scheme is designed to maximise the energy efficiency of relay-assisted D2 D communication,taking into account the influence of cellular users and non-perfect channel conditions while forming user pairs with each other.The scheme is based on a relay selection model between user pairs,and analyses the energy efficiency optimisation problem in both one-way and two-way communication between user pairs,deriving the influence of cellular users on relay selection for D2 D communication in both one-way and two-way communication,and also considering the influence of channel symmetry and asymmetry on relay selection in two-way.The optimal relay is obtained by combining the relay selection algorithm with the one that minimises the energy consumption of the whole communication system,and the corresponding algorithm is used to solve for the optimal transmission rate to obtain the best energy efficiency.The simulation results show that for both one-way and two-way D2 D communications,there is an optimal relay that forwards the signal at the best transmission rate.Simulation results also show that a symmetric bi-directional channel is more energy efficient than an asymmetric bi-directional channel at the same transmission rate. |
