This paper studies a multiple-input single-output non-orthogonal multiple access cognitive radio network relying on simultaneous wireless information and power transfer. A realistic non-linear energy harvesting model is applied and a power splitting architecture is adopted at each secondary user (SU).Since it is difficult to obtain perfect channel state information (CSI) in practice, instead either a bounded or gaussian CSI error model is considered. Our robust beamforming and power splitting ratio are jointly designed for two problems with different objectives, namely minimizing the transmission power of the cognitive base station and maximizing the total harvested energy of the SUs. The optimization problems are difficult to solve mainly because of the non-linear structure of the energy harvesting and CSI errors models. We converted them into convex forms by using semi-definite relaxation. For the minimum transmission power problem, we obtain the rank-2 solution under the bounded CSI error model, while for the maximum energy harvesting problem, a two-loop procedure using a one-dimensional search is proposed. Our simulation results show that the proposed scheme significantly outperforms its traditional orthogonal multiple access counterpart. Furthermore, the performance using the gaussian CSI error model is generally better than that using the bounded CSI error model.
Index TermsRobust beamforming, non-orthogonal multiple access, non-linear energy harvesting model, cognitive radio, imperfect channel state information.radio signals for replenishing the batteries [15]. There have been two research thrusts on EH using RF technology. One focuses on wirelessly powered networks, where a so-called harvestthen-transmit protocol is applied [16]. The other one uses simultaneous wireless information and power transfer (SWIPT) [18]-[20], which is the focus of this paper. In literature, SWIPT in CR has been extensively studied. Specifically, the work in [21] considered the optimal beamforming design in a MISO CR downlink network where a similar power splitting structure is applied in the user receiver. Hu et. al [22], on the other hand, investigated the objective of EH energy maximization, and the resource allocation problem is formulated to address that goal.Additionally, [23] considered the underlay scheme in CR network and proposed the optimal beamforming design. To address both the SE and EE, a multiple-input single-output (MISO) NOMA CR using SWIPT is considered based on a practical non-linear EH model. Robust beamforming design problems are studied under a pair of channel state information (CSI) error models. The related contributions and the motivation of our work are summarized as follows.
A. Related Work and MotivationThe prior contributions related to this paper can be divided into two categories based on the EH model adopted, i.e. the linear [20]-[36] and the non-linear EH model [16], [37]-[41]. In the linear EH model, the power harvested increases linearly with the input power, while the EH under the non-linear m...