Optimizarea performanțelor sistemelor de comunicații digitale / Optimizing the performance of digital communications systems

Wireless communication systems have evolved rapidly in terms of volume and range of services. In order to obtain high performances, numerous technologies were taken into consideration in the communication chain. This doctoral thesis aims to analyze the quality of the information that reaches the destination taking into account Orthogonal Frequency-Division Multiplexing (OFDM), channel coding (Low parity Density Check (LDPC)), the use of multiple antennas at transmission and reception (Multiple-Input Multiple-Output MIMO), the use of spread spectrum based on Code Division Multiple Access (CDMA) or based on Multi-User Shared Access - MUSA. Therefore, it starts from a MIMO system affected by various types of fading and which uses the CDMA multiple access technique when transmitting random data or images. Matlab is used as a simulation medium, and performance is analyzed by interpreting graphs that represent bit error rates (BER) versus the signal-to-noise ratio (SNR) of the Mean Structural Similarity Index (MSSIM). In addition to CDMA, OFDM technology is introduced into a system where the number of antennas at reception and the number of users varies to see what changes occur and under what conditions. The introduction of LDPC channel coding and relays follows. Transparent relays, in which the received signal is amplified, and regenerative ones, in which a series of digital processing is performed, and approached. Given the limitations introduced by CDMA, for an even greater improvement of the system developed so far, MUSA multiple access technique has been implemented, whose performance is highlighted by comparison with CDMA and whose development has started from CDMA. A new algorithm for obtaining complex spreading codes was also developed, starting from real PN spreading codes whose performances are analyzed using BER versus SNR.