  This paper considers a wireless powered communication network (WPCN) with
group cooperation, where two communication groups cooperate with each other via
wireless power transfer and time sharing to fulfill their expected information
delivering and achieve "win-win" collaboration. To explore the system
performance limits, we formulate optimization problems to respectively maximize
the weighted sum-rate and minimize the total consumed power. The time
assignment, beamforming vector and power allocation are jointly optimized under
available power and quality of service requirement constraints of both groups.
For the WSR-maximization, both fixed and flexible power scenarios are
investigated. As all problems are non-convex and have no known solution
methods, we solve them by using proper variable substitutions and the
semi-definite relaxation. We theoretically prove that our proposed solution
method guarantees the global optimum for each problem. Numerical results are
presented to show the system performance behaviors, which provide some useful
insights for future WPCN design. It shows that in such a group
cooperation-aware WPCN, optimal time assignment has the greatest effect on the
system performance than other factors.
