SCUBA: An In-Device Multiplexed Protocol for Sidelink Communication on Unlicensed Bands

Abstract

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 halfduplex 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.

Synchronized SCUBA: D2D Communication for Out-of-Sync Devices

“…The SRC obtains the IMSI and SL-DRX cycle values of the DST from a central SCUBA server, which it accesses occasionally via the P-RAT link. Please refer to [23] for more details.…”

“…Based on the regulations governing the use of different unlicensed frequency bands, we categorize SCUBA UEs into two power classes (PCs): PC1 and PC2. The physical layer specifications of the UEs in these two PCs are shown in Table I, which are derived from the geo-and band-specific regulations [23], [24].…”

“…However, the OOC and PC LTE-D2D UEs follow a synchronization approach similar to Bluetooth and Wi-Fi, where data is preceded by an SS. Since SCUBA is entirely designed by considering the underlying cellular P-RAT as the baseline protocol [23], i.e., reusing radio and chipset of the underlying P-RAT, and since SCUBA network operates similar to the LTE-D2D network under different coverage scenarios, we consider LTE-D2D as a starting point for our SCUBA sync design.…”

“…This undermines the benefits achievable with the use of SL. Therefore, a novel protocol called SL Communications on Unlicensed Bands (SCUBA) was recently developed for D2D communications in unlicensed bands [23], [24]. SCUBA operates in a time division multiplexed (TDM) manner with the underlying cellular radio access technology (RAT), e.g., long term evolution -MTC (LTE-M).…”

“…Although SCUBA solves several existing challenges for achieving SL communication on unlicensed bands for lowcost UEs, a fundamental requirement for SCUBA communication is to have the communicating UEs synchronized with each other. This is because a source UE (SRC) communicates with a destination UE (DST) by transmitting SCUBA data on a dedicated time-slot, called the SL paging occasion (SL-PO) of the DST, which it computes using a pre-defined relation [23]. To determine the exact location of the SL-PO in time and to further communicate with each other, the two UEs must be in sync with each other.…”