# Secure Information Transmission with Self Jamming SWIPT
## 1 Introduction  
In a dense wireless communication network (WCN), both the energy and security reliability need to be assured. This represents a challenge in the case of a network with a huge number of nodes [1]. Thus, in order to keep up with the energy demands of the increasing number of network nodes, new radio frequency energy harvesting (EH) techniques like simultaneous wireless information and power transmission (SWIPT) are investigated for 5G networks [1; 2; 3]. Apart from harvesting energy by using SWIPT techniques, many other signal processing applications like channel estimation with SWIPT [4] and physical layer security (PLS) measures with the help of SWIPT technique were investigated in [5]. One of the most important techniques in improving PLS in a wiretap channel is using artificial noise (AN) in the communication channel to degrade the signal-to-noise plus interference ratio (SINR) of eavesdroppers; thereby improving the security of legitimate users [6]. The high energy signal that is used for harvesting energy in SWIPT can also be adopted for creating AN to improve PLS, as in [7]. The secrecy rate of legitimate users in a wiretap channel can be improved with the SINR degradation of eavesdroppers as in [8; 9]. Furthermore, the secrecy rate for the multiple-input and multiple-output (MIMO) system model was established in [10]. To avoid the impact of AN on the legitimate receiver, AN is introduced in the null space of the legitimate receiver's channel matrix. Thereby, AN degradesthe SINR of eavesdroppers without compromising the quality of the legitimate users' signal [11]. However, the successful negation of AN is dependent on the channel state information (CSI) of the receiver at the legitimate transmitter and can be effectively used against very robust eavesdroppers as in [12; 13]. This is considered as a challenge in the imperfect CSI condition [14; 15]. Another challenge in using AN against eavesdroppers lies in the fact that a jamming attack depends on the channel correlation between the eavesdroppers and legitimate receiver; if the channel correlation is very high, then AN can be partially canceled out by the eavesdropper [16; 17].  
In this article, we use energy signals as a source of AN for secure communication, but also to harvest energy at the receiver. We consider the conventional MIMO system model with separate antennas for data and energy signal transmission and reception. AN is created by the transmitter by using channel precoding based on the CSI of the receiver. In this paper, we analyze the impact on channel precoding based on imperfect CSI. Furthermore, we also consider high channel correlation between eavesdroppers and the legitimate receiver as one of the challenges in the analysis. Our main contribution in this paper involves improving the error rate and secrecy rate performance of the legitimate receiver using iterative block decision feedback equalization (IB-DFE). IB-DFE is an efficient low complex receiver and performs better than non-iterative methods [18]. IB-DFE is effectively used with the single carrier frequency-division multiple access (SC-FDMA) transmission technique, and its performance was studied in detail [19; 20; 21]. IB-DFE can also be used to improve channel estimation at the transmitter side against imperfect CSI by using the robust superimposed pilot signal method for accurate channel estimation, as in [4; 22]. For a fair comparison between the legitimate user and eavesdroppers, we also analyze the impact of imperfect CSI on the legitimate user's performance.  
Throughout this article, matrices or vectors are denoted by bold letters, and scalar variables are denoted by italic letters. The variables associated with the frequency domain and time domain are denoted by capital letters and small letters, respectively. \((.)^{*},(.)^{T},(.)^{H},||.||\), and \(\mathbb{E}[.]\) denote the conjugate, transpose, Hermitian, trace, and expectation operation, respectively.