We consider a secondary user with energy harvesting capability. We design access schemes for the secondary user which incorporate random spectrum sensing and random access, and which make use of the primary automatic repeat request (ARQ) feedback. The sensing and access probabilities are obtained such that the secondary throughput is maximized under the constraints that both the primary and secondary queues are stable and that the primary queueing delay is kept lower than a specified value needed to guarantee a certain quality of service (QoS) for the primary user. We consider spectrum sensing errors and assume multipacket reception (MPR) capabilities. Numerical results are presented to show the enhanced performance of our proposed system over a random access system, and to demonstrate the benefit of leveraging the primary feedback.Index Terms-Cognitive radio, energy harvesting, queueing delay.

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This letter proposes a novel hybrid half-/full-duplex relaying scheme to enhance the relay channel security. A source node (Alice) communicates with her destination node (Bob) in the presence of a buffer-aided full-duplex relay node (Rooney) and a potential eavesdropper (Eve). Rooney adopts two different relaying strategies, namely randomize-and-forward and decodeand-forward relaying strategies, to improve the security of the legitimate system. In the first relaying strategy, Rooney uses a codebook different from that used at Alice. In the second relaying strategy, Rooney and Alice use the same codebooks. In addition, Rooney switches between half-duplex and full-duplex modes to further enhance the security of the legitimate system. The numerical results demonstrate that our proposed scheme achieves a significant average secrecy end-to-end throughput improvement relative to the conventional bufferless full-duplex relaying scheme.

Abstract-We propose a new artificial-noise aided hybrid time-switching/power-splitting scheme for orthogonal frequencydivision multiplexing (OFDM) systems to securely transmit data and transfer energy to a legitimate receiving node. In our proposed scheme, the cyclic prefix has two more benefits in addition to the cancellation of the inter-symbol interference between the OFDM blocks. Firstly, it enables the legitimate transmitter to send artificial-noise (AN) vectors in a way such that the interference can be canceled at the legitimate receiver prior to information decoding. Secondly, its power is used to energize the legitimate receiver. We optimize the cyclic prefix length, the time-switching and power-splitting parameters, and the power allocation ratio between the data and AN signals at the legitimate transmitter to maximize the average secrecy rate subject to a constraint on the average energy transfer rate at the legitimate receiver. Our numerical results demonstrate that our proposed scheme can achieve up to 23% average secrecy rate gain relative to a pure power-splitting scheme.Index Terms-Energy harvesting, OFDM, security, SWIPT.I. INTRODUCTION Radio-frequency (RF) energy harvesting (EH) is a powerful technology that enables wireless nodes to be charged using the ambient RF transmissions. A wireless node converts the received RF transmissions into direct current (DC) electricity. Most existing research work adopts either time-switching (TS) simultaneous wireless information and power transfer (SWIPT) schemes [1], where the receiver switches between data reception and energy harvesting, or power-splitting (PS) SWIPT schemes [1], where the receiver splits the signal into two streams of different powers for decoding information and harvesting energy separately to enable simultaneous information decoding and energy harvesting.Unlike the commonly-used pure TS or pure PS schemes for SWIPT, we propose a hybrid TS/PS scheme for wireless orthogonal frequency-division multiplexing (OFDM) systems. In [2], the authors investigated orthogonal frequency-division multiple-access (OFDMA) systems with SWIPT. Resource allocation schemes were proposed for the maximization of the energy efficiency of data transmissions. The implementation of the schemes in [2] is challenging since the authors assumed that the PS ratio can be different across the OFDM subchannels. As reported in [3], in practical circuits, (analog) power splitting is performed before (digital) OFDM demodulation. Thus, for an OFDM-based SWIPT system, all subchannels need to be power split with the same power ratio. In [3], the authors considered time-division multiple-access (TDMA) and OFDMA schemes. Each receiver applies either TS or PS to coordinate the EH and information decoding processes. In [4], the authors proposed new schemes to extend

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