Advanced spatial modulation systems for future MIMO systems

Spatial modulation (SM) seems like a promising transmission technique for multiple-input multiple-output (MIMO) systems to meet the requirements of future wireless communication technologies. SM activates only one transmit antenna in each time slot and conveys additional information bits through the indices of active transmit antennas. Thus, the spectral efficiency increases without increasing the modulation order. Besides, SM requires only one radio frequency (RF) chain, which means low-level hardware complexity and inter-channel interference (ICI) cancellation due to the activation of only one transmit antenna. In recent years, the design of SM-based transmission techniques has become a general research topic because of the advantages of SM, and it is seen that clever SM-based designs can increase the spectral efficiency and bit error rate (BER) performance concerning SM. In this thesis, three novel SM-based transmission schemes are proposed for next-generation MIMO systems. Firstly, a novel transmission scheme named transmit antenna grouping quadrature SM (TAG-QSM) is proposed. The motivation of TAG-QSM is to increase the number of information bits in the spatial domain to achieve lower modulation order than the existing schemes in the literature. In TAG-QSM, transmit antenna groups with an equal number of transmit antennas are created. After that, first, the incoming bit group determines the indices of transmit antenna groups separately for the transmission of the real and imaginary parts of data symbol. The second incoming bit group chooses one transmit antenna from each of the determined transmit antenna groups. Data symbol is determined using the last bit group. To conclude, the increase in the number of additional information bits in the spatial domain results in lower modulation order and improved BER performance concerning the benchmark schemes. In this study, the theoretical upper bound is derived for BER, and Monte Carlo simulations are demonstrated for comparing the BER performance of TAG-QSM with the benchmark schemes and showing the consistency of the theoretical upper bound of BER derivation. Secondly, flexible spatial modulation with transmit antenna selection (FSM-TAS) is introduced for future multiple-input multiple-output (MIMO) systems. In this scheme, the number of active antennas varies in each time slot depending on the incoming bits. After determining the number of active antennas, channel coefficients corresponding to each possible active antenna combination are added up. Then, a certain number of antenna combinations with largest gains is selected to apply spatial modulation (SM). For the proposed system, complexity and outage probability analyses are performed. In addition, it has been shown by Monte Carlo simulations that FSM-TAS provides better bit error rate (BER) performance than the benchmark scheme, named enhanced spatial modulation with generalized antenna selection (ESM-GAS) \cite{qing2021enhanced}, under the same spectral efficiency, the same number of transmitter and receiver antennas. Thirdly, spatial modulation (SM) using signal space diversity (SM-SSD) is proposed for multiple-input multiple-output (MIMO) systems. In this scheme, consecutive time slots are processed jointly and signal and space diversity (SSD) technique is applied for the transmission of data symbols to obtain transmit diversity. In addition, a clever active antenna activation algorithm is introduced to prevent transmit diversity degradation. An upper bound BER derivation is performed and compared with Monte Carlo simulations. Besides, BER performance comparison is demonstrated with the reference schemes in the literature. Lastly, the suboptimal solution for the rotation angles is expressed to maximize minimum coding gain distance (MCGD).