Study on wireless energy harvesting in cooperative communications with relay selection and physical layer security

Abstract

Recently, Wireless Energy Harvesting (WEH), in which wireless nodes power their batteries by scavenging energy from ambient Radio Frequency (RF) signals, has become a promising solution to address the challenge of prolonging the lifetime of energy-constrained wireless networks and reducing the periodic recharging and replacement of batteries. Due to the great advantages mentioned above, the WEH technique finds important applications in both point-to-point communication and Cooperative Communication (CC), where energy-constrained relays harvest energy from wireless signals sent by the Access Points (APs) to assist the communication between the APs and their destinations. More recently, security in WEH has become an emerged research topic which has attracted a lot interest from researchers. Because of the fact that the WEH nodes usually locate near the AP for harvesting much energy, they are capable of overhearing the confidential information from the AP. To overcome this problem, the Physical Layer Security (PLS) technique, which is capable of providing secure communication without using cipher codes, has become an effective solution. Motivated by above, in this thesis we focus on investigating two aspects of WEH in the cooperative relaying systems: relay selection and PLS. Moreover, we extend our studies by considering the effects of hardware impairment and imperfect channel on the (secrecy) performance of the WEH CC systems. The (secrecy) performance of all proposed systems is evaluated via mathematical analyses. The accuracy of the analytical results is verified by Monte Carlo simulations.

We first study the problem of K-th best relay selection in WEH system by analyzing a communication between a multiple-antennas source-destination pair via a K-th best relay of a single-antenna WEH relay network. Moreover, we propose deploying an energy beamforming technique known as Maximal Ratio Transmission (MRT) at the source and a combining technique known as Maximum Ratio Combining (MRC) at the destination for these such systems to improve the system performance.

Then, we extend our first study to the scenario of WEH CC system with non–ideal hardware. We assume that the RF impairments caused by the RF font-end hardware imperfections are not completely removed by using signal processing algorithms; hence, the Residual Transmit RF Impairment (RTRI) notably degrades the system performance. From the results in terms of the Outage Probability (OP) and capacity, we show that the influence of RTRI on the system performance can be effectively mitigated by using more antennas and/or more relays.

Next, we focus on investigating PLS in WEH CC systems. Different with most studies on PLS in CC, where external eavesdroppers create security risk, our studies deal with the problem of PLS in WEH CC using untrusted relays. In this investigation, we focus on studying the effects of imperfect channels and transmit-antenna techniques, i.e., MRT, Transmit Antenna Selection (TAS) and Random Antenna Selection (RAS), on the secure communication of the proposed system.

After that, we study the secure communication of a CC system via an untrusted WEH relay in the presence of an external eavesdropper. The secure communication can be overhear by both the untrusted relay and the external eavesdropper. In this problem, we focus on investigating the effects of the locations and the transmit powers on the secrecy performance. The obtained results show that in the presence of the external eavesdropper, a reasonable location of the untrusted relay can yield a higher secrecy performance.

Finally, we study the problem of joint relay–selection and PLS in an untrusted relaying WEH system. We propose employing multiple antennas and the MRT technique at the source and destination. Then, we use the MRT–based relay–selection methods to select the relay for assisting the communication. These proposals allow us to boost the harvested energy at the selected relay; moreover, under the assumption that lacking global knowledge of Channel State Information (CSI) at the relays, they also allow us to eliminate the security risks from the non–selected relays.