  Internet of Everything (IoE) applications such as haptics, human-computer
interaction, and extended reality, using the sixth-generation (6G) of wireless
systems have diverse requirements in terms of latency, reliability, data rate,
and user-defined performance metrics. Therefore, enabling IoE applications over
6G requires a new framework that can be used to manage, operate, and optimize
the 6G wireless system and its underlying IoE services. Such a new framework
for 6G can be based on digital twins. Digital twins use a virtual
representation of the 6G physical system along with the associated algorithms
(e.g., machine learning, optimization), communication technologies (e.g.,
millimeter-wave and terahertz communication), computing systems (e.g., edge
computing and cloud computing), as well as privacy and security-related
technologists (e.g., blockchain). First, we present the key design requirements
for enabling 6G through the use of a digital twin. Next, the architectural
components and trends such as edge-based twins, cloud-based-twins, and
edge-cloud-based twins are presented. Furthermore, we provide a comparative
description of various twins. Finally, we outline and recommend guidelines for
several future research directions.
