JOINT ENERGY BEAMFORMING AND TIME ASSIGNMENT FOR COOPERATIVE WIRELESS POWERED NETWORKS

Abstract

Wireless power transfer (WPT) and radio frequency (RF)-based energy har-

vesting arouses a new wireless network paradigm termed as wireless powered com-

munication network (WPCN), where some energy-constrained nodes are enabled to

harvest energy from the RF signals transferred by other energy-sufficient nodes to

support the communication operations in the network, which brings a promising

approach for future energy-constrained wireless network design.

In this paper, we focus on the optimal WPCN design. We consider a net-

work composed of two communication groups, where the first group has sufficient

power supply but no available bandwidth, and the second group has licensed band-

width but very limited power to perform required information transmission. For

such a system, we introduce the power and bandwidth cooperation between the

two groups so that both group can accomplish their expected information delivering

tasks. Multiple antennas are employed at the hybrid access point (H-AP) to en-

hance both energy and information transfer efficiency and the cooperative relaying

is employed to help the power-limited group to enhance its information transmission

throughput. Compared with existing works, cooperative relaying, time assignment,

power allocation, and energy beamforming are jointly designed in a single system.

Firstly, we propose a cooperative transmission protocol for the considered system,

where group 1 transmits some power to group 2 to help group 2 with information

transmission and then group 2 gives some bandwidth to group 1 in return. Sec-

ondly, to explore the information transmission performance limit of the system, we

formulate two optimization problems to maximize the system weighted sum rate by

jointly optimizing the time assignment, power allocation, and energy beamforming

under two different power constraints, i.e., the fixed power constraint and the aver-

age power constraint, respectively. In order to make the cooperation between the

two groups meaningful and guarantee the quality of service (QoS) requirements of

both groups, the minimal required data rates of the two groups are considered as

constraints for the optimal system design. As both problems are non-convex and

have no known solutions, we solve it by using proper variable substitutions and the

semi-definite relaxation (SDR). We theoretically prove that our proposed solution

method can guarantee to find the global optimal solution. Thirdly, consider that

the WPCN has promising application potentials in future energy-constrained net-

works, e.g., wireless sensor network (WSN), wireless body area network (WBAN)

and Internet of Things (IoT), where the power consumption is very critical. We

investigate the minimal power consumption optimal design for the considered co-

operation WPCN. For this, we formulate an optimization problem to minimize the

total consumed power by jointly optimizing the time assignment, power allocation,

and energy beamforming under required data rate constraints. As the problem is

also non-convex and has no known solutions, we solve it by using some variable

substitutions and the SDR method. We also theoretically prove that our proposed

solution method for the minimal power consumption design guarantees the global

optimal solution. Extensive experimental results are provided to discuss the system

performance behaviors, which provide some useful insights for future WPCN design.

It shows that the average power constrained system achieves higher weighted sum

rate than the fixed power constrained system. Besides, it also shows that in such a

WPCN, relay should be placed closer to the multi-antenna H-AP to achieve higher

weighted sum rate and consume lower total power.