Abstract-A harvest-use-store power splitting (PS) relaying strategy with distributed beamforming is proposed for wirelesspowered multi-relay cooperative networks in this paper. Different from the conventional battery-free PS relaying strategy, harvested energy is prioritized to power information relaying while the remainder is accumulated and stored for future usage with the help of a battery in the proposed strategy, which supports an efficient utilization of harvested energy. However, PS affects throughput at subsequent time slots due to the battery operations including the charging and discharging. To this end, PS and battery operations are coupled with distributed beamforming. A throughput optimization problem to incorporate these coupled operations is formulated though it is intractable. To address the intractability of the optimization, a layered optimization method is proposed to achieve the optimal joint PS and battery operation design with non-causal channel state information (CSI), in which the PS and the battery operation can be analyzed in a decomposed manner. Then, a general case with causal CSI is considered, where the proposed layered optimization method is extended by utilizing the statistical properties of CSI. To reach a better tradeoff between performance and complexity, a greedy method that requires no information about subsequent time slots is proposed. Simulation results reveal the upper and lower bound on performance of the proposed strategy, which are reached by the layered optimization method with non-causal CSI and the greedy method, respectively. Moreover, the proposed strategy outperforms the conventional PS-based relaying without energy accumulation and time switching-based relaying strategy. Index Terms-Wireless-powered communication, power splitting, harvest-use-store, channel state information.  The fifth generation (5G) communication networks are expected to support new emerging services with high network capacity, as well as a reduced delay and energy consumption. To achieve these requirements, the use of super-dense small cell deployments and centralized resource management, i.e. cloud radio access network, is becoming an appealing approach [1], [2] . However, to fulfill the desired coverage, some wireless nodes in the 5G network might need to be deployed in places lacking an external power supply. To this end, energy harvesting approaches that scavenge energy from the ambient environment are recognized as a key enabling technology for these self-sustainable nodes [3] . Meanwhile, cooperative relay communication is a promising approach to enlarge coverage and improve spectral efficiency. Therefore, enabling cooperative relay communications via energy harvesting is becoming a popular concept for green communication, which aims at decreasing power usage, while improving the transmission performance. A key concern of the energy harvesting enabled cooperative relay communication is the efficient utilization of harvested power, which is not steadily replenished as in traditional grid-aided communication networks. The issue of improving transmission performance via an efficient utilization of harvested power has been widely studied for conventional energy harvesting techniques, where natural resources, such as solar, wind etc. are used as energy sources [4], [5] . However, the intermittent and unpredictable nature makes these sources difficult to exploit in certain environments. As an alternative to the conventional energy harvesting techniques, radio-frequency (RF) energy harvesting techniques are believed to fully unleash the potential gains of energy harvesting, in which RF signals transmitted from the source node can be used as energy sources for cooperative nodes. Moreover, it has been illustrated in [6] that wireless-powered cooperative relay communications can be realized within a boundary distance, which is determined by both the available transferred energy and the minimum energy requirements of the harvesting devices. As a result, the wireless-powered cooperative relay communication is formed and can be regarded as a promising solution to highly energy-efficient networking. Note that, due to the dual-purpose of RF signals,
In REF , Zhou et al. proposed a multislot EH-based PSR strategy with distributed beamforming for wireless powered multi-relay cooperative networks.
Wireless-Powered Cooperative Communications: Power-Splitting Relaying with Energy Accumulation
IEEE Journal on Selected Areas in Communications
IEEE Journal on Selected Areas in Communications
Computer Science Mathematics