Majed Saad defended his PhD thesis on Tuesday, December 15 at CentraleSupélec. The research leading to these results received funding from the French National Research Agency (ANR-17-CE25-0013) within the frame of the BRAVE project, CentraleSupélec, Lebanese International University (LIU), and Centre national de la recherche scientifique (CNRS)-France.

Wireless Terabits per second (Tbps) link is needed for the new emerging data-hungry applications in Beyond 5G (B5G) (e.g., high capacity broadband, enhanced hotspot, three Dimensional (3D) extended reality, etc.). Besides, the sub-THz/THz bands are the next frontier for B5G due to scarce sub-GHz spectrum and insufficient bandwidth for wireless Tbps link in 5G millimeter Wave (mmWave) bands. Even though a wider bandwidth and large-scale Multiple-Input Multiple-Output (MIMO) are envisioned at sub-THz bands, but the system and waveform design should consider the channel characteristics, the technological limitations, and the Radio Frequency (RF) impairments. Based on these challenges, we proposed to use an energy-efficient low order single carrier modulation and then increase its spectral efficiency by Index Modulation (IM) and MIMO techniques. This thesis is divided into two main parts, and both follow this research methodology.

In the first part, MIMO Spatial Multiplexing (SMX) and spatial IM domain (e.g., Generalized Spatial Modulation (GSM)) are investigated and studied in the context of the sub-THz environment with RF impairments. Then, we tackled their main challenges related to detection complexity and the performance degradation of GSM in highly correlated channels. Hence, we proposed a quasi-optimal detector for each system with a complexity reduction in the order of 99%. Afterward, we proposed two techniques to reduce the spatial correlation effect on GSM without instantaneous channel side information at the transmitter. We also proposed Dual-Polarized GSM (DP-GSM) that provides higher Spectral Efficiency (SE) gain via multi-dimensional IM and helps with the latter problem by exploiting the polarization IM dimension.

In the second part, we proposed a novel IM dimension, called filter IM domain, that generalizes most existing Single-Input Single-Output (SISO)-IM schemes with/without IM. Within the filter IM domain, we proposed two novel schemes: Filter Shapes Index Modulation (FSIM), In-phase (I) & Quadrature (Q) Filter Shape Index Modulation (IQ-FSIM) to enhance the system SE and Energy Efficiency (EE) through indexation of the filters in the bank. These schemes convey information bits by indexing the selected pulse shaping filter and transmitting complex data symbol. However, the former uses the same filter shape for the I &Q components, while the latter performs independent indexation for the I & Q components to double the FSIM SE gain. These schemes achieve higher SE gain than most of the existing SISO systems with/without IM due to exploiting all available time/frequency resources and changing the indexed filter at the symbol rate.
Starving for further SE and EE improvement, this filter IM domain is exploited in MIMO by proposing an SMX-FSIM system with a simple linear and parallel receiver architecture. Besides, the optimal low complexity detectors and the specialized equalization techniques for the three systems are designed. Also, the performance of these systems is theoretically characterized. Finally, the challenging filter bank design problem imposed by the filter IM domain is tackled by optimization to achieve better results.

To conclude, the proposed SMX-FSIM is compared in the sub-THz environment to the previously considered candidates. The results confirm that the non-coherent SMX-FSIM is a very promising and competitive solution for a low-power wireless Tbps B5G system due to its high SE and EE, robustness to RF impairments, low power consumption, low cost, and feasible complexity with a simple linear receiver.

For more details, the readers can refer to [1] or [2]

[1] M. Saad, N. Al Akkad, H. Hijazi, A. Chamas Al Ghouwayel, F. Bader and J. Palicot, “Novel MIMO Technique for Wireless Terabits Systems in sub-THz Band,” in IEEE Open Journal of Vehicular Technology, vol. 2, 2021.

[2] Majed Saad, Back to Single-Carrier for Beyond-5G Communications above 90GHz: Novel Index Modulation techniques for low-power Wireless Terabits system in sub-THz bands, Ph.D. dissertation, CentraleSupélec, Jan. 2021.