 network. Recently, authors of [135, 138] and [238] studied optimal resource allocation problems in CR associated with SWIPT. In [138], an optimal transmit beamforming scheme was proposed in a multi-objective optimization framework. It was shown that there are several tradeoffs in CR-aided SWIPT. Based on the work in [138], the authors proposed a jointly optimal beamforming and power splitting scheme to minimize the transmit power of the base station in multiple-user CR-aided SWIPT [135]. Considering the practical imperfect CSI, Zhou et al. [236] studied robust beamforming design problems in MISO CR-aided SWIPT, where the bounded and the gaus- sian CSI error models were applied. It was shown that the performance achieved under the gaussian CSI error model is better than that obtained under the bounded CSI error model. The work in [236] was then extended to MIMO CR-aided SWIPT in [54] and [213], where the bounded CSI error model was applied in [54] and the gaussian CSI error model was used in [213] and [98]. In contrast to [135-173, 178, 180-194, 196-213], Zhou et al. [238] studied robust resource allocation problems in CR-aided SWIPT under opportunistic spectrum access. 5.1 Background 5.1.1.2 Non-linear EH Model In [22], robust resource allocation schemes were proposed for maximizing the sum trans- mission rate or the max-min transmission rate of MIMO-assisted wireless powered com- munication networks, where a practical non-linear EH model is considered. It was shown that a performance gain can be obtained under a practical non-linear EH model over that attained under the linear EH model, see Figure 4.7. In order to maximize the power-efficient and sum-energy harvested by SWIPT systems, Boshkovska et al. designed optimal beam- forming schemes in [24] and [20]. Recently, under the idealized perfect CSI assumption, the rate-energy region was quantified in MIMO systems relying on SWIPT and the practi- cal non-linear EH model in [212]. In order to improve the security of a SWIPT system, a robust beamforming design pr