  Device-to-device (D2D) communication is an essential component enabling
connectivity for the Internet-of-Things (IoT). SCUBA, which stands for Sidelink
Communication on Unlicensed Bands, is a novel medium access control protocol
that facilitates D2D communications on the sidelink for IoT and machine-type
communication (MTC) cellular devices. SCUBA includes support for direct
peer-to-peer communication on the unlicensed bands by operating in a time
division multiplexed manner to coexist with the underlying primary radio access
technology, e.g., long term evolution - MTC (LTE-M). A fundamental requirement
in the current version of SCUBA is that the communicating devices are to be
synchronized with each other so that timing occasions of the devices can be
accurately estimated by each other. However, when the devices are out of sync
with each other, which may be caused due to one or more of the devices being
out of cellular coverage region or are being served by different base stations,
typically observed in mobile devices, operation of legacy SCUBA fails. To this
end, we design synchronization methods to establish successful SCUBA links
between devices that are out-of-sync with each other. Due to the inherent
timing discovery embedded in our method, our solution also extends the
operating range of SCUBA by eliminating its reliance on timing-agnostic
communication. We analyze and compare the performance of our methods in terms
of power consumption and the resultant impact on device battery life to show
the potential of our solutions.
