Pulse Shaping for Terahertz Communications

Abstract

Over the last two decades, terahertz communications has emerged to unleash the potential of the electromagnetic spectrum beyond 100 GHz. The rapid development in this area has derived benefits from the state-of-the-art breakthroughs in terahertz technology. However, employing this technology to terahertz communications applications comes with a number of interrelated technical challenges, which tend to ceil the overall performance of terahertz communications systems. On a fundamental level, the information-bearing waveforms routed over these systems are power-limited due to the inherently low radiation levels of currently available terahertz transmitters and the availability of terahertz amplifiers. In addition, globally adopted wireless standards introduce additional spectral power and bandwidth constraints to these waveforms. Hence, the prudent utilisation of the permissible regulatory emission power raises a key challenge. Another important challenge is the reliable decoding of these waveforms after wireless transmission. At the receiver front-end, the signal-to-noise ratio of terahertz signals becomes severely low, making them highly sensitive to time-jittering and hence, prone to decoding errors, especially at high baudrates. Further to that, simplifying the transceivers architectures in future wireless communications without sacrificing the high throughput is another fundamental design challenge, which is usually foreseen as an implicit urging necessity for nontraditional waveforms with high spectral efficiency. Concurrently, there is a pressing need for hardware-based computationally efficient signal processing techniques. This challenge is driven by high bit rates available from terahertz communications, leading to an increased latency in digital post-processing. Despite their importance, the aforementioned design challenges remain under-investigated. Therefore, specifically designed waveforms and systems able to sustain in such severe transmission conditions are sought after. To this end, the main research theme of this dissertation focuses on pulse shaping in terahertz communications. Specifically, two pulse shapes are proposed based on the Lorentzian pulse. The first pulse shape, referred to here as the logarithmic Lorentzian pulse, fully complies with the spectral constraints imposed by the IEEE Standard for terahertz communications and possesses a close-to-unity spectral radiation efficiency. The spectrum of this pulse is designed based on the partial overlap of two logarithmic Lorentzian spectra spaced apart by the desired bandwidth, leading to a flat, i.e., frequency-invariant, in-band spectrum and controllable out-of-band suppression. Consequently, an additional degree of freedom is granted to this pulse in order to flexibly control the inter-band interference and hence, adapt to different multi-band transmission scenarios. Validated by rigorous experiments, this pulse shape shows a potential to support terahertz signalling with a limited emission power and stringent spectral constraints, compared to the conventional pulse shapes. Under the same experimental conditions, a proposed variant of this pulse is designed and tested. The modified pulse shows a high tolerance over a wide range of the time-jittering at the receiver side. This outcome further emphasises the vital role of pulse shaping for terahertz communications. Furthermore, the optical communications-inspired carrierless amplitude and phase modulation technique is adopted to terahertz communications. Experimental results show that this technique is a spectrally efficient orthogonal pulse shaping and modulation contender that can significantly simplify the architecture of terahertz transceivers for the sixth generation (6G) terahertz communications systems and, at the same time, achieve a high throughput. Signal processing techniques for terahertz communications constitute a supplementary research theme in this dissertation. In this part, a microwave photonics-based pulse shaping technique for multi-band terahertz communications is proposed and experimentally demonstrated. Volterra nonlinear filters are also employed to compensate for the dispersion and high nonlinearity of photonics-based terahertz communications systems with optical and wireless transmission. Additionally, a low complexity equalisation technique for multiple-input multiple output single-carrier terahertz communications is proposed and simulated using experimental measurements for the indoor terahertz channel frequency response. The adoption of the concepts and contributions presented throughout this dissertation can significantly improve the performance of existing terahertz communications systems. Additionally, the proposed pulse shapes establish the foundations for further development of other terahertz-specific waveforms and can be used as informing guidelines for the design and development of terahertz communications systems with high spectral power efficiency as well as robustness to time-jittering under the limited power emission levels offered by current terahertz technology. Moreover, the signal processing techniques presented in this dissertation can potentially cope up with the high data rate requirements of terahertz communications.