Enhancing Connectivity and Transmission Robustness for Vehicular Communications Assisted by SGF NOMA and Full-Duplex Relaying

This paper merges the advantages of full-duplex (FD) multi-antenna cooperative relaying and non-orthogonal multiple access (NOMA)-aided semi-grant-free (SGF) transmission schemes to fulfill the requirements of lower latency, higher reliability and a greater number of connected devices in the vehicular networks. To achieve these objectives, a low complexity and cost-efficient hybrid NOMA-OMA-based resource allocation mechanism is introduced which can enhance network connectivity without aggravating interference. The hybrid successive-interference-cancellation (SIC) decoding method is utilized to improve transmission robustness. Additionally, a novel hybrid SIC decoding approach is specifically designed when the transmitted signals experience the same fading effects over a common channel transmission mode, i.e. the link between the relay and the base station (BS). We show that the FD relay-assisted vehicular communication system suffers from an outage probability error floor. Hence a modified zero-forcing (ZF) criterion-based transmit beamformer is designed to enhance the transmission robustness and eliminate the residual self-interference (SI) caused by FD operation. Moreover, a truncated Gaussian distribution is considered to model vehicle mobility. In order to minimize the line-of-sight (LoS) link obstruction phenomenon, the optimal vehicle speed is derived. Further, the exact and asymptotic outage probability expressions are derived, based on which the analysis of diversity order is provided. To validate the accuracy of our analytical results, Monte-Carlo simulations are conducted. Our results reveal that the hybrid SIC-based decoding scheme for SGF NOMA transmission provides a superior outage performance compared to the conventional NOMA transmission with the conventional SIC-based decoding scheme. Additionally, combining the hybrid SIC decoding method with the relay beamforming approach can eliminate the outage probability error floor while guaranteeing the quality of service (QoS) for the primary vehicle in the uplink FD cooperative vehicular communication system.