# Secrecy Enhancing Scheme for Spatial Modulation Using Antenna Selection and Artificial Noise
## 1 Introduction  
Spatial modulation (SM) is a spatial multiplexing multiple-input multiple-output (MIMO) scheme, and it is known that SM can overcome the drawbacks of the conventional MIMO techniques [1]. For example, SM can entirely avoid the inter-channel interference (ICI) and inter-antenna synchronization (IAS) problems by activating only a part, usually one of the transmit-antennas for data transmission at any signaling time instance. In addition, it usually needs one radio frequency (RF) chain for data transmission [2]. Due to the above-mentioned advantages, SM is expected to be one of the key technologies for future wireless networks [3]. SM is considered as a three-dimensional (3D) modulation technique, because it conveys information bits by utilizing both the antenna index and complex symbols to form a 3D constellation [4]. With this concept, efficient 3D constellation schemes were proposed in order to achieve a better spatial multiplexing gain and high data throughput [4, 5]. It was emphasized that they could be easily extended to a massive MIMO system by reducing the cost of the massive numbers of RF chains at the base stations, and the security scheme could be directly adopted. On the other hand, efficient antenna selection techniques were proposed to enhance the system performance. For example, a Euclidean distance optimized antennaselection (EDAS) method was proposed to offer a significant signal-to-noise ratio (SNR) gain [6], and later, the achievable transmit diversity order of the EDAS was analyzed [7]. Due to the inherent broadcasting nature of wireless communication systems, security and privacy protection comprise an increasingly important issue in the design and implementation of wireless networks. Recently, physical layer security (PLS) schemes combined with MIMO techniques have gained much attention, especially when SM is used [8]. Because SM changes the active antenna dynamically, it can be easily applied to a PLS scheme. For example, an antenna selection scheme for SM was proposed to achieve PLS transmission [9]. In combination with the antenna selection, the idea of injecting jamming signals, i.e., artificial noises (AN), into the transmission of SM was proposed to enhance the security [10]. It was presented that activating an extra transmit antenna to transmit AN can increase the security of SM. A similar fact was discussed in [11], where the authors proposed a secure transmission scheme for differential quadrature spatial modulation (DQSM) with AN, and it was shown that the AN technique was a meaningful idea to improve the secrecy performance. Because the AN can be also interference to the legitimate receiver, the antenna carrying AN needs to be directed to the null space, and thus, additional beamforming technique was required. One of our previous studies presented a PLS scheme with AN, which did not require a beamforming technique [12]. On the other hand, a symbol rotation-based secrecy-enhancing scheme combined with SM was proposed [13]. In this scheme, complex symbol values for transmit antenna indices and information were dynamically rotated, where the rotation values were optimized for legitimate channel state information (CSI). However, all of the aforementioned schemes assumed perfect CSI knowledge at least for the legitimate channel. By considering that the influence of imperfect CSI due to the channel estimation error cannot be ignored, this paper proposes a new secrecy-enhancing scheme for SM under imperfect CSI. We adopt the idea of combining the antenna selection with AN for PLS and utilize the additional activation of the antenna only for transmitting AN. Especially, we propose a method to employ two AN sequences, which have the co-cancellation property, and thus, they can perfectly cancel each other only at the legitimate receiver. We assume the imperfect CSI condition by considering the practical system operation condition, and thus, the generation of AN and antenna selection are all optimized by considering imperfect CSI.