This article presents the results of a study on the noise immunity of DVB channels when higher-order M-ary APSK modulation schemes and concatenated BCH-LDPC codes are used. Dependencies to determine the probability at the decoder output are given taking into consideration the BCH and LDPC code parameters and the error probability in the communication channel. The influence of the BCH packets length, the BCH code rate, the number of maximum iteration and the parameters of LDPC parity-check matrix on the code efficiency is analyzed. Research of the influence of the concatenated LDPC-BCH code parameters on the radio channel noise immunity is conducted and dependencies to determine the required CNR at the input of the satellite receiver are given.

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The paper deals with some investigations aimed at applying compensation techniques to reduce nonlinear signal distortion in the Mach-Zehnder modulator of HFC CATV systems. The frequency distribution of second-and third-order intermodulation products of N-channel transmission systems is shown. Models of dual-parallel and dual-cascade Mach-Zehnder modulators are suggested based on both the mathematical model of a conventional Mach-Zehnder modulator and the features of the CATV system itself, thus making it possible for the modulator parameters to be determined in a way to ensure CSO products cancellation and CTB products minimization. The dependence of the C/CTB parameter on the modulation index is studied for N-channel HFC CATV systems applying a dual-parallel or dual-cascade Mach-Zehnder modulator. Optical channels implementing dual-parallel or dual-cascade Mach-Zehnder modulator instead of a conventional one are compared and the possibilities to improve the RF signals' dynamic range are analyzed. Relations to determine the increase of the optical loss introduced by dual Mach-Zehnder modulators are suggested that help compare it with that caused by a conventional Mach-Zehnder modulator.