# Hiding Stealth Optical CDMA Signals in Public BPSK Channels for Optical Wireless Communication
## 1 Introduction  
With the increasing application of computers in different areas of life and work, information security has become an important concern. To enhance security in the physical layer of an optical network, several approaches have been investigated, such as quantum private communication, optical encryption, and optical steganography [1; 2; 3; 4; 5; 6]. Steganography is one of the methods that have received attention in recent years. The word steganography, which is derived from Greek, literally means \"covered writing\". The main goal of steganography is to hide information sufficiently well such that any unintended recipients do not suspect that the steganographic medium contains hidden data [7]. This is a major distinction between steganography and the other methods of improving security. For example, optical encryption allows a signal to be encrypted with low latency; thus, the recipient requires a key to read the information. However, each person notices the information by seeing the coded signal; that is, the method cannot prevent the signal from being detected. In some cases, the system is already in danger of being decrypted if an eavesdropper knows about the existence of the signal. Steganography provides an additional layer of security by hiding the data transmission underneath the steganographic medium. The physical layer security has become an important issueof communication security technique because it is cost-efficient without sacrificing much data rate. It takes the advantages of channel randomness nature of transmission media to achieve communication confidentiality and authentication [8; 9; 10]. Steganography operations have been performed on different cover media such as images, audio, text, and video [11]. In recent years, new research into methods for secure communication over existing public fiber-optic networks has been conducted. Using a concept similar to steganography, the secure signal is processed by a particular technique called optical steganography and can be hidden under the noise floor of a public network. Optical steganography is realized by transmitting a private signal hidden in the existing public channel to increase the secrecy of the communication system. One of the first methods using the concept of optical steganography was proposed by [PERSON] and [PERSON] [12; 13]. In this method, the signal is hidden using spread spectrum techniques. Several researchers have worked with various secret transmissions over different types of public network. For example, [PERSON] (2007) proposed a method of secure stealth transmission over a wavelength-division multiplexing (WDM) network [14]. [PERSON] (2008) discussed coherent spectral-phase-encoded OCDMA signal transmission in a WDM network [15]. [PERSON] (2011) investigated stealth transmission over a public differential phase-shift keying (DPSK) channel [16]. [PERSON] and [PERSON] (2014) demonstrated an optical steganography technique based on amplified spontaneous emission noise [17]. The basic approach of optical steganography is to temporally stretch the pulses of a signal using high-dispersion elements. The amplitudes of the signal pulses are dramatically decreased after the stretching so that the signal can be hidden underneath the public signal and system noise. However, once the existence of the hidden channel is revealed, eavesdroppers may take measures to process the stealth signal; therefore, the stealth channel may be weak against eavesdropping. To enhance the security of the stealth channel, additional security measures are required against eavesdroppers. For this, optical code-division multiple-access (OCDMA)--which has been successfully applied in optical steganography--is a great choice [17; 18; 19; 20]. The research uses the techniques of polarization modulator based code-shift-keying (CSK) data modulation for OCDMA codec. The wavelength selective switch is used for optical en/decoding and it shows the stealth signals can transmitted over a 40-km wavelength-division multiplexing optical fiber link [21].