In the vehicular ad hoc networks, a slotted ALOHA-based scheme for effective broadcast services is essential to disseminate the control and safety packets since it can avoid the slot collision issues. Existing slotted ALOHA-based schemes, for example, VeMAC, simply abandon the conflicting slots for all the collided nodes, which leads to a high collision probability and low wireless resource utilization. To address this issue, a logically centralized slotted ALOHA-based (LC-SA) scheme is proposed in this article. Different from the traditional slotted ALOHA-based schemes, the proposed LC-SA scheme solves the slot release problem and improves the rate of successful channel access. In the LC-SA scheme, a control node is enabled to reserve time slots for its neighbor nodes. In this way, a collided node within the communication range of the control node will continue to acquire this time slot if a collision is implied, while other collided nodes out of range will release this time slot to prevent another collision. Simulation results in both perfect and imperfect channel conditions are presented. The LC-SA scheme is verified to have higher throughput than VeMAC, due to more a rapid channel access speed and fewer access collisions.
In this paper, we consider safety message transmission in a dense vehicular network. With increasing vehicular network density, the collision rate increases when multiple vehicles transmit safety messages simultaneously. To address this issue, we propose a request-transmission split time division multiple access (TDMA) scheme, referred to as RTS-TDMA. In our scheme, we divide a frame into three phases, i.e., a contention access phase, a broadcast feedback phase, and a contentionfree transmission phase. Each vehicle selects a repetition rate according to a given probability distribution and repeats the transmission of its request packet to improve the reliability of the request. In addition, a roadside unit acts as the coordinator and uses a successive interference cancellation technique to resolve request collisions. RTS-TDMA also reduces the request time percentage by containing only the vehicle identity in each request packet. Both theoretical analysis and numerical results verify that the RTS-TDMA scheme can provide higher throughput than the coded slotted ALOHA scheme.
In the existing slotted ALOHA-based access schemes for vehicle-to-everything (V2X) networks, the duration of a time interval for competitive users contains the whole process of requesting and transmission. To improve the access capacity for V2X networks, unlike these schemes, we propose a request-transmission splitting slotted ALOHA-based scheme, referred to as the RTS-SA, in which a frame is divided into three phases. The first phase is short and utilized by vehicles when competitively transmitting their identities to roadside units (RSUs) for accessing requests in a coded manner. Then, RSUs implement multiuser detection and decoding simultaneously and act as the coordinator to broadcast the allocated time slots for the vehicles to guarantee contention-free transmission in the third phase. Compared to the VeMAC scheme, the RTS-SA can improve vehicles' successful access performance compared to the VeMAC scheme, which is indexed by increasing the throughput. Simulation results verify the effectiveness of our proposed scheme. INDEX TERMS Access scheme, slotted ALOHA, roadside unit (RSU), successive interference cancellation, vehicle-to-everything (V2X). YONG LIANG GUAN received the bachelor's degree (Hons.) from the National University of Singapore and the Ph.D. degree from Imperial College London, U.K. He is currently a tenured Associate Professor with the School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore, where he also leads two industry collaboration labs: NTU-NXP Smart Mobility Lab and Schaeffler Hub for Advanced Research (SHARE), NTU. His research interests include coding and signal processing for communication and data storage systems.
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