  Sixth-generation (6G) of mobile communication aims to connect this world
digitally through green communication networks that provide secure, ubiquitous,
and unlimited connectivity in an attempt to improve the overall quality of
life.The driving force behind the development of these networks is the rapid
evolution of Internet-of-Things (IoT), which has stimulated the proliferation
of wireless applications in health,education, agriculture, utilities, etc.
However, these applications are accompanied by the deployment of a massive
number of IoT devices that require a significant radio spectrum for wireless
connectivity. IoT devices usually have low data rate requirements and limited
power provision but desirably a long life.Recently, the development of passive
radio systems has opened new paradigms of spectrum sharing and coexistence.
These systems utilize the radio resources and infrastructure of the active
radio systems to perform their functionalities. By enabling the dependent
coexistence, a new technology named symbiotic radio (SRad) enables the
symbiotic relationships between the different radio systems ranging from mutual
benefits or competition in terms of sharing the resources, in particular for
IoT devices. This survey first provides the motivation for dependent
coexistence and background of spectrum sharing through coexistence along with
existing literature. Then, it describes the active and passive radio systems
definition and a brief overview. Afterward, the history of symbiosis and the
role of SRad technology in spectrum sharing and coexistence are defined while
focusing on symbiotic communication. Lastly, we discuss research challenges,
future directions, and applications scenarios.
