Analysis of M state digitally modulated signals in communication systems based on SDR concept

The speed at which new digital communication systems are being developed is increasing dramatically. Growing quantity of transmitted information and growing quantity of users require development of modern communication systems based on new principles; existing systems are quickly becoming obsolete technologically. Digital radio and television broadcasting (DVB-T, DVB-T2), which break down into three options — satellite, cable and terrestrial systems expanded massively in the Czech Republic in 2010. Multistate digital modulation M-QAM is used for transmission of information in these systems. This article is focused on the analysis of digitally modulated signals in communication systems which use software defined radio concept and modern synthetic instruments. Authors of this article present real results, which illustrate the link between number of symbols in M-state QAM modulation, SNR and BER. Constellation diagrams of transmitted and received symbols (with superposition of noise) were presented in the article. Simple picture is transmitted through simulated radio channel to show the result of signal impairments. Experiments were done using software defined radio concept of communication system. Modular PXI HW platform was used in connection with graphically oriented development environment. This combination of modular HW and flexible SW components allows changing the communication protocol, modulation scheme, frequency bandwidth and other parameters in a very simple way by changing the software part of the system.