Nonterrestrial networks (NTNs) complement the terrestrial cellular networks by extending coverage to the user equipments (UEs) located in unserved and underserved areas. One of the most critical problems faced by NTN UEs is the lack of uplink (UL) synchronization and the associated initial access failure resulting from the high Doppler frequency offset caused by satellite velocity. While NTN new radio (NR) UEs rely on the global navigation satellite system (GNSS) to resolve the UL synchronization problem, it is not always feasible for power-critical NTN Internet of Things (IoT) UEs. To this end, we design synchronization signal-based positioning in IoT NTNs (SPIN) which enables the IoT UEs to tackle the UL synchronization problem. SPIN estimates position and velocity of the UE using time difference of arrival (TDOA) and frequency difference of arrival (FDOA) measurements on the downlink synchronization signals. Consequently, the UEs can use the position and velocity estimates to compute and compensate for the residual time and frequency offsets, thereby successfully synchronizing to the NTN UL. We conduct physical layer and system-level simulations to show the effectiveness of our solution. SPIN positioning accuracy achieves the Cramér-Rao lower bound and meets the target accuracy required for UL synchronization. We also compare the battery life of an NTN IoT UE which uses SPIN for UL synchronization with that of a UE using GNSS-based solution. Our numerical results show that SPIN has significant battery life savings over GNSS-based solution while also maintaining a low computational complexity.

Device-to-device communication (D2D) is a key enabler for connecting devices together to form the Internet of Things (IoT)[1]. 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 multiplexed 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.

Minimizing the resynchronization time in Long-Term Evolution for Machine Type Communications (LTE-M) is highly desirable to enhance the battery lifetime of low-power and possibly deep-coverage user equipments (UEs) targeting Internet-of-Things (IoT) applications. This has led the 3rd Generation Partnership Project (3GPP) to initiate a work item in Re1-15 LTE-M to improve cell search and/or system information acquisition time. This has been accomplished by developing a new resynchronization signal (RSS) that can be detected during relatively short device wake-up times and with low computational effort. In this paper, we review the design aspects that have been considered while deriving the new RSS signal and describe the RSS design that has been adopted by 3GPP. Moreover, we propose another RSS design and show the differences between the proposed design and the one adopted by 3GPP in terms of the detection performance and the computational and memory complexity. Finally, we highlight the gains resulting from employing the new RSS design compared to the legacy synchronization signals.

Device-to-device (D2D) communication finds widespread applicability in enabling connectivity for the Internet of Things. Leveraging the unlicensed frequency bands for D2D communication reduces the spectrum licensing costs, and offloads network traffic from the limited licensed spectral resources. Towards achieving a cost-efficient solution, we study the feasibility of integrating such a D2D operation in the unlicensed band (D2D-U) within legacy cellular devices as a secondary radio access technology in a time division multiplexed manner. To this end, we evaluate the constraints faced in incorporating D2D-U in various challenging environments, such as within devices that use a single radio architecture and a half-duplex with frequency division duplexed cellular operation. We propose time instances that can be exploited to multiplex D2D-U without violating regulatory restrictions imposed on using different unlicensed frequency bands. Based on our analyses and simulation results, we suggest guidelines for a prospective D2D-U protocol that can achieve efficient sidelink communications while also ensuring coexistence with legacy cellular operation.

Machine Type Communications (MTC) is one of the major drivers for the future growth of the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE)/LTE-Advanced (LTE-A) standards. A primary challenge associated with MTC using LTE/LTE-A is to cater to varied requirements of the User Equipment (UE) like coverage enhancement, low complexity, low power consumption, etc. The current LTE/LTE-A standards incorporate the Discontinuous Reception (DRX) mechanism for reduced energy consumption where the UE wakes up periodically to check for a paging message from the base-station. On each wake-up occasion, the UE has to decode the paging information, which requires accurate timing resynchronization. In the case of the MTC UEs, especially when the devices are deployed in low coverage areas, the resynchronization consumes a significant amount of time which increases the ON time of the UE leading to higher energy consumption. In this paper, we introduce the Enhanced Primary Synchronization Signal (ePSS) within the LTE/LTE-A standardization framework and propose a novel DRX mechanism which uses our ePSS for faster resynchronization and reduced energy consumption. We demonstrate that our mechanisms are more energy efficient than the legacy DRX mechanism for the MTC UEs with coverage enhancement.

The Third Generation Partnership Project (3GPP) has recognized machine-type communications (MTCs) as a vital medium to drive the Internet of Things (IoT). One of the key challenges in MTC is to reduce the energy consumption of the MTC user equipment (UE). Currently, the 3GPP Long Term Evolution (LTE)/LTE-advanced (LTE-A) standards incorporate discontinuous reception (DRX) mechanism for this purpose. In this paper, we propose a modified DRX mechanism incorporating the quick sleeping indication (QSI) as a novel, simple, and energy-efficient solution for low-complexity, low-mobility MTC UEs. We demonstrate our QSI transmission mechanisms using the broadcast and synchronization channels of LTE/LTE-A for MTC UEs in normal coverage and using the data channel for MTC UEs operating in “coverage enhancement” (CE) mode. For MTC UEs in normal coverage, our simulation results and analysis show that our DRX with QSI mechanisms result in 45% improvement in the energy efficiency and 66% reduction in the computational complexity at the UE receiver, when compared to the current DRX mechanism. For MTC UEs with CE, the energy and computational efficiency increase to 63% and 68%, respectively.