Enhancement Techniques for OTFS and Hybrid Reflection Modulation

Abstract

Orthogonal time frequency space (OTFS) modulation and hybrid reflection modulation (HRM) are emerging technologies in wireless communications. OTFS provides robustness in high Doppler channels. HRM improves rate and performance in reconfigurable intelligent surfaces (RIS) aided systems. This thesis proposes techniques to enhance the rate and performance in OTFS and HRM systems. The thesis is organized into two parts. The first part focuses on OTFS, and the second part focuses on HRM.

The first part of the thesis focuses on improving the performance of OTFS by integrating channel modulation (CM) and index modulation (IM) in the delay-Doppler (DD) domain. Initially, we propose a novel OTFS-CM scheme, where CM is used to enhance OTFS performance. In CM, radio frequency (RF) mirrors placed in the near field of the transmit antenna are controlled by information bits, which create different channel fade realizations that form the CM alphabet. The proposed OTFS-CM scheme is shown to simultaneously offer the benefits of robustness to high-Doppler due to DD signal processing in OTFS and an additional signal-to-noise ratio (SNR) gain due to the improved distance properties of the signal set due to CM. We further improve the performance by incorporating delay-Doppler indexing in the OTFS-CM framework. We consider two types of DD indexing, one with constellation indexing and one without. Simulation results show that the proposed OTFS-CM-IM scheme performs better than the traditional OTFS scheme without CM and IM, which is attributed to the improved distance properties of the proposed OTFS-CM-IM signal set.

The second part of the thesis explores HRM schemes that integrate active and passive reflecting elements in RIS-aided systems to enhance the rate and performance. By employing active elements capable of signal amplification along with phase tuning, the HRM approach effectively mitigates the double-fading effect inherent to RIS-aided systems. Additionally, the choice of the elements to be made active conveys information bits through indexing, with additional bits conveyed through symbols from a conventional modulation alphabet. In this part, we propose methods to design improved HRM signal sets that achieve better bit error performance. We propose a signal set design procedure that efficiently packs non-equidistant signal points on the positive real line when all active elements use the same amplification factor. We further improve this signal set by obtaining the optimum amplification factors that maximize the minimum Euclidean distance of the signal set. Simulation results show improved bit error performance of the proposed HRM signal sets compared to the basic HRM signal set.