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Quantum-based satellite free space optical communication and microwave photonics

Spread-spectrum techniques are widely used in radio communication and telecommunication. Any signal like acoustic, electrical, and electromagnetic signal produced with a specific bandwidth is spread in frequency hence results in a wider bandwidth. Spread spectrum techniques are deployed in telecommunication because of many significant advantages, e.g., to achieve secure communications, to detect the eavesdropping, to resist natural interference, to bound power flux density for satellite down links and resistance to noise and jamming. In spread spectrum technique, frequency hopping (FH) is used as a basic modulation technique by which any telecommunication signal can be transmitted on a wider bandwidth (radio bandwidth) as compared to frequency value of the original signal. Spread spectrum techniques deploy FH, direct sequence (DS), or mix of both methods so that it can be used for multiple access and reduces the interference to other receivers to get the overall privacy. At the receiver side, the received signals are correlated to extract the original information being sent. The two main motivation behind spread spectrum are: to create anti-jamming for unauthenticated person and to provide low probability of interception. The classical data transmission can also be achieved by using quantum communication protocols like dense coding, quantum key distribution (QKD), and quantum teleportation. Most of these protocols use quantum states of light to transmit the information through optical fibers. The problem of channel access become apparent when number of users increases. In classical communication networks, when many users want to use the same channel at the same time, many multiple access methods are used. On the other side, quantum communication networks also deploy frequency and wavelength division multiple access techniques, hence each user make use of same channel at different frequencies and different time instance. In addition to this, many multiple access techniques employ orbital angular momentum of single photons and coherent states. Quantum spread spectrum multiple access scheme described here for optical fibers could be used for free-space data transmission. In this scheme, multiple users send their photons via same optical fiber, sharing their path, time window, and frequency band. A number of techniques have been proposed previously for data transmission for quantum optical communication which provides heavy losses while combining and separating the transmitted data from different users. The described scheme is developed to follow classical spread spectrum methods, and also follows add-drop architecture with simple extraction and combination points.