Performance Analysis of Sidelink Relay in SCMA-based Multicasting for Platooning in V2X Jingwei Fu, Gang Wu, and Ran Li The National Key Laboratory of Science and Technology on Communications, University of Electronic Science and Technology of China, Sichuan, China. Corresponding author: Gang Wu (e-mail: wugang99@uestc.edu.cn). Abstract—This work presents a sparse code multiple access (SCMA)-based multicasting scheme for platooning in vehicles communication. In order to extend the coverage range of the platoon leader, we consider that the platoon members can be utilized as decode-forward (DF) relays. The single relay scheme and the multi-relay scheme are proposed in this paper. The average outage probabilities under the two relay strategies and direct transmission are analyzed, and the explicit expressions are derived. In addition, this paper analyzes the total communication cycle of the two relay strategies in a multi-hop transmission scenario and compare their delay performances. The simulation results prove that the utilization of relay can greatly reduce the outage probability of the system. The single relay scheme has the best outage performance among three transmission schemes since the optimal relay is selected. Moreover, although the multi-relay scheme sacrifices the outage performance by introducing inter- user interference, it can reduce the total communication cycle time and meet the low-latency communication requirement. Index Terms—platooning, multicast, SCMA, relay, outage probability, delay. I. I NTRODUCTION In recent years, more and more intelligent services are provided for end-users by using co-operative awareness, which means that knowledge of different entities environment can be processed and shared to meet service requirements such as autonomous driving. Thus, an integrated system for ve- hicle networks is proposed, namely the vehicles-to-everything (V2X) network. The V2X network mainly ensures the commu- nication of vehicle-to-vehicle (V2V), vehicle-to-infrastructure /network (V2I/N), and vehicle to pedestrian (V2P) [1]. Five application scenarios in V2X have been defined in [2], and vehicles platooning is one of them worth studying. In platooning scenario, a group of autonomous/semi- autonomous vehicles move on the same lane in a closely linked manner like a train. The group contains one platoon leader (PL) and multiple platoon members (PMs). The first vehicle of a platoon is called the PL, it is in charge of platoon manage- ment and control. The other vehicles in the platoon, namely the PMs, will travel under the guide of PL [3]. PL is supposed to send different messages to PMs using the same spectrum resource to improve spectrum efficiency. But this kind of signal superposition will cause interference between users. Fortunately, non-orthogonal multiple access (NOMA) offers an This paper is supported in part by Innovation Fund of National Key Lab of S&T on Communications and the National Science Foundation of China (Grant No. 61771106). effective solution. Based on the user specific non-orthogonal signatures, NOMA can control the inter-user interference. At receiver, NOMA can separate the superimposed user signals by employing a deliberately designed multi-user detection (MUD) receiver and interference cancellation (IC) technology [4]. Thus the combination of NOMA and multicast can better satisfy the communication requirements of platooning scenario [5]. Since the number of PMs in a platoon is usually fixed, sparse code multiple access (SCMA) [6] is a promising candi- date among the several NOMA schemes. In SCMA encoder, the incoming bit streams are mapped to sparse codewords. Benefiting from the sparsity of SCMA codewords, the low complexity parallel decoding algorithms, such as the message passing algorithm (MPA) introduced by [7] and [8], can be utilized as the receiver algorithm. Compared with other receivers, SCMA receivers not only have greater maximum likelihood (ML)-like decoding performance, but also can avoid error propagation [9]. Thus, we proposed a scenario that the PL multicasts a superimposed signal encoded by SCMA to the PMs and uses the MPA receiver. However, the signal transmitted by PL will experience a large path loss fading when the platoon is too long, causing the distant PM to fail to decode the PL signal correctly. Relaying technology is an effective way to overcome path loss. Sidelink relay is also proposed as a new work item in Release 17 [10]. Moreover, many papers have proposed that PM can forward PL signals as a decode-forward (DF) relay to maintain the quality of platooning communication. The relay selection scheme based on the the reception probability has been proposed in [11], but it introduced more latency. In [12], the forwarding and reverse link quality has been taken into consideration. But the performance is affected by the realistic measured channel impairment. The scheme in [13] overcame the predicted link quality limitation with the proposition of a dynamic link quality (BDSC) estimation algorithm. While, this scheme is more suitable for unicast routing because of the definition of the reverse link. In this paper, we proposes two relay schemes, namely single relay scheme and multi-relay scheme. We assume that all of the relays mentioned in this paper are DF relays with time division duplex (TDD) mode. Different from the existing works, neither of the two relay schemes requires link quality measurement, the relay selection is based on geographic loca- 978-1-7281-7440-2/20/$31.00 ©2020 IEEE