# Wideband Mixed Signal Separation Based on Photonic Signal Processing
## 1 Introduction  
High-speed communications boost economic growth by supporting a wide range of applications that are changing the way that Americans live, including the internet of things, unmanned vehicle systems, radars for transportation, and cyber physical systems, to name a few. Private communication protects civilians, as well as empowers homeland security, by preventing confidential information from being exposed and are increasingly important in a connected world. Therefore, the availability of high-speed communications and whether the communication is protected from eavesdropping are two key considerations of building a communication network. Interference management has been widely deployed to enable the availability of wireless communication channels [1], and stealth communication hides the existence of the signal and protects users' privacy in the physical layer [2; 3]. Both interference management and stealth communication require the processing and separation of the signal of interest (SOI) from interference and noise in real-time, while the growing needs of communication dramatically increase the bandwidth of the signals and thus create an increasing challenge to the contemporary framework of signal processing. This review introduces the current Interference management and stealth communication systems, the challenges, and the feasibility of hybrid systems that address the growing needs of wideband real-time signal processing by leveraging the mutual benefits of photonic signal processing. Optical communication uses light to transfer information, and photonic signal processing technique uses optical spectrum to analyze, modify, and synthesize, which are inherently compatible with fiber-optic communication systems. Photonicmethods have been demonstrated to directly process the wideband analog the signal with zero latency [4; 5; 6]. By using photonic methods to remove interference, the bandwidth and dynamic range of the signal are greatly reduced for the digital edge device to process. On the other hand, the photonic methods require strict matching conditions between physical parameters to achieve optimized signal separation. Such a matching condition can be easily identified in a point-to-point stationary link [7], while in a network, the physical parameters, such as wireless channel coefficients, private keys to recover the stealth signal, scales up with the number of nodes in the network and is changing over time. Such a change cannot be tracked simply with the photonic system. By using the edge device to dynamically measure the statistical pattern of the mixed signal, the change in the physical parameters can be followed, and the edge device can accurately control the photonic system to achieve the matching condition and perform signal separation. The capability of separating wideband signals impact communication availability and confidentiality through two fundamentals, while not yet solving problems in communications: interference management and stealth communication. The core of both problems is to separate the SOI from interference and noise. Interference management: Separation SOI from unknown signals, also known as blind source separation (BSS), enables multiple wireless systems to share the same radio frequency (RF) spectrum at the same time. As a scarce resource, the RF spectrum is simultaneously being used with multiple wireless communication protocols and thus interference is generated between systems [8; 9]. The bandwidth and the power dynamic range of the separation system determine whether multiple wireless systems can coexist in the same band. Stealth communication: Separation SOI from pre-known signals or intentionally added noise enables stealth communication. A stealth method, such as spread spectrum technique, prohibits eavesdropper access by hiding signals in wideband noise, while it also increases the difficulties of authorized users with the right keys [10; 11]. A large amount of data needs to be processed to separate the signal from the wideband noise.