Classification of signals, their characteristics, examples of use in communication systems. Continuous-time periodic and non-periodic signals, Fourier transforms, their properties. Discrete-time signals, Discrete Fourier Transforms, digital filtering, FIR and IIR filters. Modulation, AM, FM, PM signals, digital modulation, transformation of modulated signals by narrowband linear systems. Random signals, their main parameters, principles of measurement, noise in electronic systems.