Energy Efficiency Comparison of THz and VCSEL-Based OWC for 6G

Abstract

Both optical wireless communications (OWC) and Terahertz (THz) communication systems are regarded as enablers for the 6th-generation (6G) networks. Facing the 6G requirements of ultra-dense and high-rate networks, vertical-cavity surface-emitting laser (VCSEL)-based OWC is considered as a strong contender for providing high data rates with ultra-small cells. Hence, this work quantitatively compares VCSEL-based OWC and THz communication in terms of energy efficiency. We derive the energy efficiency model for realistic systems by including non-linearity power conversion of VCSEL and area-bandwidth tradeoff of the photodiode (PD) for OWC, and gain and power loss in cascaded THz systems, respectively. Then, we formulate the similarity of embedding optical lenses into VCSEL-based system and high-directivity antenna into THz system to achieve high channel gain, i.e., leading to a gain-coverage tradeoff for both systems. Taking account of this performance tradeoff in analyzing two systems' energy efficiency, we show numerically the superiority of the VCSEL-based system, while the THz system is more robust in long-range scenarios due to a more relaxed relationship between pointing error and transmission distances. This study provides design guidelines for selecting appropriate technologies tailored to various 6G application scenarios from an energy efficiency perspective.