Advanced MIMO Techniques for Future Wireless Communication Systems

The rapid growth of wireless communication systems has led to an increasing demand for higher data rates, better reliability, and more efficient use of available resources. This thesis is dedicated to exploring three advanced technologies that have the potential to address these challenges in future wireless communication networks. These technologies are non-orthogonal transmission (NOT), spatial modulation (SM), and reconfigurable intelligent surfaces (RISs). The thesis presents innovative signal processing algorithms, as well as transmitters and receivers that incorporate these technologies. The goal is to enhance the performance of wireless communication systems by leveraging the advantages of these emerging technologies. In recent years, NOT has drawn significant attention in wireless communications. Many studies have explored the use of NOT in the frequency and time domains in order to develop multiple access techniques to enhance the spectral efficiency of wireless networks. However, applying NOT to the spatial domain has not been thoroughly investigated. This thesis builds a new architecture for precoding in a multiple-input multiple-output (MIMO) system based on singular value decomposition (SVD), called sparse layered MIMO (SL-MIMO). On the other hand, RIS technology is set to become an essential component of future wireless communication networks which can manipulate the propagation channel to improve signal quality at the receiver. One promising application is RIS-assisted spatial modulation (SM) systems, which provide a new approach for beyond 5G (B5G) networks. However, prior research has only explored the use of basic SM with a single active antenna in RIS-assisted communication systems, where each group of RIS elements targets only one transmit/receive antenna. This thesis proposes novel architectures for RIS-assisted SM systems in which two or more antennas are selected for the purpose of SM. Specifically, RIS-assisted receive quadrature space shift keying (RIS-RQSSK) is introduced, which enhances the spectral efficiency of an RIS-based index modulation (IM) system by using the real and imaginary dimensions independently. Therefore, the error rate performance of the system is improved as all RIS elements reflect the incident transmit signal toward both selected receive antennas. To improve the spectral efficiency of the RIS-RQSSK system, an extension is proposed in this thesis that incorporates conventional in-phase/quadrature (IQ) modulation alongside spatial modulation. This new system is called RIS-assisted receive quadrature spatial modulation (RIS-RQSM). We propose a novel methodology to adjust the phase shifts of the RIS elements in order to maximize the signal-to-noise ratio (SNR) and at the same time to construct two separate PAM symbols at the selected receive antennas, as the in-phase and quadrature components of the desired IQ symbol. To complete this research, a generalized architecture for the RIS-assisted SM system is proposed, in which multiple receive antennas are targeted to enhance system performance. The proposed system is called RIS-assisted generalized receive quadrature spatial modulation (RIS-GRQSM). Finally, we propose multicast MIMO communications which can be considered as a special case of the RIS-GRQSM system. The numerical results show that the proposed schemes notably outperform state-of-the-art benchmark schemes in terms of error rate performance.