At present, the transfer of information is an integral part of technologies that are actively developing in the framework of the process called the Fourth Industrial Revolution. In this, space-satellite, satellite and other mobile wireless communication systems play an increasingly important role. Almost all of them include multiple access, which means a method of common resource division of the communication channel between subscribers (each mobile station has the ability to use a satellite retransmitter or the base station of a mobile wireless communication system to transmit its signals regardless of the operation of another station). Multiple-access communication systems are used for digital radio and television broadcasting in high-speed communication lines, in wireless local area networks, for data transmission in the microwave range, and also for communication with various mobile partners. In the radio transmitting and receiving paths of communication systems with multiple access, multiple signals are used (the sum of the power of the subscriber signals) with very complex types of digital envelope modulation, so they use wide working bands. With an increase in the quality of information transmission in mobile wireless communication systems, there are special requirements for powerful amplification systems (PAS) of receiving-transmitting tracts, which must have high efficiency and high output power, required bandwidth, network capacity, and linearity of message transmission channels. To achieve maximum efficiency in the PAS, the operating point of its amplifying element should be near the saturation region, on the main nonlinearity of the transfer characteristic. When multiple signals are introduced simultaneously into the PAS, it generates unfiltered intermodulation harmonics (IH). Intermodulation harmonics are formed due to the nonlinearity of the amplitude characteristics and the unevenness of phase-amplitude characteristics and due to the need to work with the highest efficiency of the PAS, which requires a shift of the operating point to the saturation thresholds of their amplifying elements. This, in turn, causes the appearance of IH. Since the harmonic oscillations IH actually represent noise for neighboring communication channels and are not theoretically filtered, an equalizer (otherwise an optimizer) of characteristics, is needed to reduce the level of these interferences in the output (Raman) spectrum of the PAS.
Physical-Layer Security Improvement with Reconfigurable Intelligent Surfaces for 6G Wireless Communication Systems
