Abstract
Due to exponential growth in mobile data traffic, intelligent devices, and connectivity, there is an ever-increasing demand for higher spectral/energy efficiency and more reliable communications. To this end, sixth generation (6G) networks will require to deploy more advanced modulation techniques than the existing ones. In this regard, index modulation (IM) has gained considerable popularity thanks to their attractive advantages, including low-complexity implementation, better error performance, and high spectral/energy efficiency (SE/EE) over conventional modulation and multiplexing schemes. Although IM is a potential candidate for the next-generation networks due to its beneficial properties, one needs to design more reliable, spectrally efficient and flexible modulation techniques to satisfy the requirements of the next-generation networks. Therefore, the overall objective of this thesis is to investigate reliable and spectral/energy-efficient IM-based techniques, especially spatial domain IM, i.e., spatial modulation (SM). Firstly, we design a general golden angle modulation (GAM) aided fractional SM (GAM-FSM) to facilitate rate matching and rate adaption in FSM. Meanwhile, a series of constellation shaping optimization problems are formulated to further enhance the system performance. Secondly, a novel combinatorial tool aided IM, namely composition aided generalized quadrature SM (C-GQSM), is proposed to improve the SE of the conventional GQSM systems by exploiting the power domain degree of freedom. And two low-complexity detectors are designed. Thirdly, aiming at addressing the pilot contamination problem in uplink channels, a novel differential SM is utilized to realize non-coherent detection. To further obtain the diversity gain, a differential SM transmission scheme of space-time block coding (STBC) is designed, and a low-complexity block-by-block signal detection algorithm is derived. Finally, we first incorporate the concept of IM into simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) aided non-orthogonal multiple access (NOMA) system to enhance the SE. Specifically, the proposed IM aided STAR-RIS-NOMA system enables extra information bits to be transmitted by allocating subsurfaces to different users in a pre-defined subsurface allocation pattern. An approximate closed-form expression on average bit error rate (ABER) is derived, which is validated by the Monte Carlo simulations.