Research On Joint Beamforming Design For Wireless Powered Relay Network

With the rapid development of radio frequency(RF)-based energy harvesting(EH),wireless power transfer(WPT)has been a promising approach to prolong the lifetime of batter charged or energy limited devices,and thus.WPT has been drawn much afftention in wireless communcations.Wireless powered relay network is one of the very important applications among the WPT-based communications systems.In the wireless powered relay network,the relays or users can act as energy sources or energy constrained devices,where the energy sources take charge of transmitting energy-carrying signals,and the energy constrained devices harvest the RF energy which will be supporting the information transmisstion.WPT not only can enable energy constrained devices working more reliably and continuously,but also makes these devices deployed flexibly due to no cable connected or batteries replacements.The wireless powered relay networks can be adopted in many practical scenarios,such as wireless internet of things networks,cellular networks,and ad hoc networks.Joint resource allocation is one of the key issues in the wireless communications,and thus,designing efficient allocation schemes can ensure the balanced usages.In addition,the system performance can be significantly improved.In this thesis,the joint beamforming optimization for the wireless powered relay networks will be the key issue.Considering the practical applications,new system architectures will be constructed,and then,the coordination mechanism of the system is also analyzed.The goal is to maximize the throughput or minimize the total transmit power by designing joint resource allocation.The formulated optimization problems are usually highly non-convex,and thus,it is difficult to solve.To tackle this challenge,efficient algorithms will be proposed to solve those optimization problems.Finally,numerical results are presented to show the advantages of the proposed schemes.The main contributions of this thesis are stated as follows.1.Joint beamforming design and time allocation for a two-way EH relay network is studied In order to efficiently power the energy constrained relays,an additional multiantenna power beacon is deployed nearby.Unlike the existing studies that assumed the symmetric two-way relaying scenario,a more practical scenario of asymmetric two-way relaying considers different traffic demands on the downlink and uplink.The goal is to design the optimal beamforming vectors at the relays and at the power beacon jointly with the time allocation between information and EH transmissions,in the sense of maximizing the sum rate of the asymmetric information transmission under the EH constraints at the relays,and the power constraint at the power beacon.The formulated optimization problem is nonconvex,and thus,it is difficult to solve.To solve this challenging problem,an iterative algorithm based on the semidefinite relaxation and the successive convex approximation techniques is proposed.Also,to reduce the computational complexity,an efficient noniterative scheme based on the maximum-ratio transmission is developed.The performance of the proposed schemes is demonstrated through simulations.2.Joint beamforming optimization and time allocation for a wireless powered two-way hybrid-relay network is investigated.As a general relaying architecture accommodating the existing relaying architectures in the literature as two special cases,the two-way communication system where both EH relays and non-EH relays exist is considered.The goal is to maximize the minimum rate by jointly optimizing the distributed beamforming vector at the relays,the energy beamforming matrix at the non-EH relays,and the EH time allocation with the energy constraints at the relays.The formulated problem is intractable to solve because the objective function and the energy constraints are highly non-convex.In order to tackle this challenge,two algorithms are proposed using the successive convex approximation and alternating optimization.Simulations demonstrate that the proposed iterative algorithm considerably outperforms any existing approaches3.Joint beamforming optimization for a wireless powered multi-pair two-way relay network(MP-TWRN)is studied.Different from the existing works,relay beamforming and power-domain non-orthogonal multiple access for the wireless powered MP-TWRN is investigated together.The goal is to optimize the energy-transfer beamforming matrix and the relaying beamforming matrix in the sense of maximizing the minimum of the achievable rates among all the users with the power constraint at the relay.Further,the above issue are explored under two relaying schemes:amplifsy-and-forward and decode-and-forward.The formulated problems are highly non-convex,and thus,it is challenging to solve.To address these,two iterative algorithms to solve those two problems based on the difference of convex programming transformation and the successive convex approximation are proposed.Simulations demonstrate that the proposed schemes outperform the OMA schemes.4.Joint source and relay beamforming for a wireless powered downlink multi-relay multi-user network is studied.Considering non-linear energy harvesting and imperfect channel state information,the goal is to minimize the total transmit power at the base station by jointly optimizing the source beamforming and the relay beamforming weights under the energy causality constraints at the relays and the signal-to-noise ratio constraints at the users.The formulated problem is highly non-convex,and thus,it is difficult to solve.To solve the problem,we first transform it into a worst-case optimization,and then,an iterative algorithm is developed to solve this worst-case optimization.Numerical results show the advantage of the proposed robust scheme.