  Device-to-device communication (D2D) is a key enabler for connecting devices
together to form the Internet of Things (IoT). A growing issue with IoT
networks is the increasing number of IoT devices congesting the spectral
resources of the cellular bands. Operating D2D in unlicensed band alleviates
this issue by offloading network traffic from the licensed bands, while also
reducing the associated licensing costs. To this end, we present a new low-cost
radio access technology (RAT) protocol, called Sidelink Communications on
Unlicensed BAnds (SCUBA), which can be implemented on cellular devices such
that it coexists with the legacy cellular protocol by operating as a secondary
RAT in a time division duplex manner using the existing radio hardware. SCUBA
is compatible on different types of cellular devices including the
low-complexity half-duplex frequency division duplex machine type communication
(MTC) user equipments. SCUBA provides flexible sidelink (SL) latency and
battery life tradeoff using a discontinuous reception procedure, which ensures
that it is applicable across a wide range of use cases. We prove the
effectiveness of our protocol with analyses and simulation results of the
medium access control layer of SCUBA using different types of MTC traffic for
both SL and the underlying cellular communication.
