Physical layer design for ultrabroadband terahertz communications: from theory to experiments

Abstract

Terahertz (THz)-band (0.1 THz to 10 THz) communication is envisioned as a key technology to meet the demand for faster, more ubiquitous wireless communication networks for the sixth generation (6G) of wireless systems and beyond. For many years, the lack of compact, fast and efficient ways to generate, modulate, detect and demodulate THz signals has limited the feasibility of such communication systems. Recent progress within different device technologies is finally closing the terahertz technology gap and enabling, for the first time, experimental wireless research in the THz band. This thesis presents the first steps towards advancing the development and bridging the gap between theoretical and experimental THz communication research. At the core of this work, the TeraNova platform, i.e, the first testbed for ultra-broadband wireless communications at THz frequencies in the world, is designed and built. In terms of hardware, the platform consists of multiple sets of analog front-ends at three different frequencies between 100 GHz and 1.05 THz and three different digital signal processing back-ends, able to manipulate tens of GHz of bandwidth. In terms of software, tailored framing, time synchronization, channel estimation, and single and multi-carrier modulation techniques are implemented in guided by the experimental characterization of the THz hardware and the THz channel. Moreover, implementation details and early experimental results to demonstrate the platform's capabilities/limitations are reported. The platform is then used to demonstrate several milestones in the field, including the first true THz link in the first absorption-defined window above 1 THz (i.e., 1-1.05 THz) and the longest multi-kilometer link (2.01 Km) at the 200-240 GHz band. Further, Knowing the peculiarities of the THz band and the available device technology in the frequency range, innovative solutions are proposed. Based on the observed behaviors, M-ary amplitude and phase-shift keying is presented to simultaneously overcome the limitations due to peak to average power ratio (PAPR) and reduce the effective symbol error rate (SER), both while using a high-order modulation scheme. This prototype is crucial for creating links able to support 100 Gbps and moving towards tera-bits-per-second (Tbps) links. Based on the unique molecular absorption at THz frequencies, two innovative modulation schemes are presented to make the most of the THz channel. First, to not only overcome but exploit the distance-dependent bandwidth of the THz band, hierarchical bandwidth modulations are proposed as a suitable candidate for a single transmitter and multiple receiver (STMR) system. Here, the number of users is multiplexed depending on their demodulation capability based on the received signal power as well as the available bandwidth. Second, to reliably communicate even in the presence of absorption peaks, chirp spread spectrum (CSS) based communication is investigated. Specifically, Chirp-Spread Binary Phase-Shift Keying (CS-BPSK) is proposed over traditional Binary Chirp Spread Spectrum (BCSS) to obtain better BER. Moreover, beyond the physics, current spectrum allocations break down the otherwise very large bands into narrow sub-sets to accommodate sensing users. Spectrum sharing is needed to make the most out of the spectral resources. Therefore, the capability of the direct sequence spread spectrum (DSSS) is explored to illustrate the performance by acknowledging the coexistence between active and passive users. Further, channel study/sounding is conducted in diverse indoor and outdoor scenarios to understand the channel statistics and design reliable communication links. Remarkably several channel models have been recently proposed at different frequency ranges within the THz band, which were specific to the environments and capturing scenarios, and, moreover, many times, these are narrowband characterizations (missing the point of THz networks). Instead, as the last contribution in this dissertation, the channel model and statistics are explored for an ultra-broadband outdoor channel in different weather conditions. Further, the channel metrics are explored in various indoor scenarios with different structural and geometrical aspects, occupancy, antenna gain, and atmospheric conditions at 130 GHz to realize the probability distribution as well as the correlation of the metrics such as delay spread, angular spread, and path loss coefficient. For this purpose, a fully tailored signal processing backend for sliding correlator type channel sounder is developed, which is capable of capturing multipath profiles with high resolution and dynamic range to describe the ultra-broadband nature of the link. In a nutshell, this dissertation presents the technologies and the results, highlights the challenges, and defines a path to move forward with innovative solutions toward practical THz ultra-broadband and long-distance communication systems.--Author's abstract