# Research on Hybrid Relay Protocol Design and Cross-Layer Performance Based on NOMA
## 1 Introduction  
With the construction and development of smart grids, energy Internet of Things, and other applications, the number of terminal devices and service types are increasing, which requires future communication networks to achieve larger terminal access, stronger signal coverage performance, and a higher communication rate under the condition of limited spectrum resources [1, 2, 3]. NOMA can significantly improve the spectral efficiency and user fairness of the network [4, 5]. NOMA-based wireless communication access and power line communication relay technology can not only improve the flexibility of network use and the overall performance of communication systems, but also have important theoretical research value and application prospects in scenarios such as remote automatic meter reading in power, ubiquitous power Internet of Things, and smart home. NOMA is a multi-access technology that enables users to communicate with non-orthogonal resources at the cost of high receiver complexity [6]. It allows users to share the same channel resources with different channel power levels, uses overlay coding technology to achieve power domain multiplexing at the transmitting end, and uses sequential serial interference cancellation (SIC) technology to separate signals at the receiving end [4, 5]. Because of its high spectral efficiency, it has attracted extensive attention in recent years. Ref. [6] studied the performance of a decoded and forwarded relay double-hop hybrid communication system based on NOMA, and derived the closed expressions of outageprobability, asymptotic outage probability, and traversal capacity. Ref. [7] proposed a continuous user relay (SR) to improve the spectral characteristics of cooperative NOMA systems. By using a pair of near users to relay the signals of far users, the interrupt fairness and spectrum utilization between near and far users are significantly improved. [PERSON] et al. [4] studied an energy-efficient resource allocation technique for hybrid Time division multiple access (TDMA)-non-orthogonal multiple access (NOMA) systems. With the goal of maximizing the overall energy efficiency of the system under the constraints of minimum rate and transmission power per user, the available time slots and available transmission power are jointly allocated. In [5], the NOMA technology was applied to the downlink multi-relay cooperative Power Line Communication (PLC) network. In order to meet the destination node quality of Service (QoS) requirements and minimize the system power consumption, a distributed opportunistic relay selection scheme based on weighted harmonic mean was designed for Decode-and-Forward (DF) and Amplify-and-Forward (AF) protocols, respectively. The designed scheme can obtain better system performance than the benchmark scheme. Existing research on NOMA technology mainly focuses on the physical layer or MAC layer of wireless and power line communication, and rarely analyzes from the perspective of cross-layer. It is shown that cross-layer analysis is critical for performance prediction, design, and optimization of protocols. Based on the downlink heterogeneous network NOMA system, [PERSON]. [8] proposed a long-term cross-layer resource allocation model with dynamic traffic arrival and limited channel information. In the case of low-overhead single-bit feedback, the optimal decoding sequence, user scheduling and power allocation were analyzed. Ref. [9] studied the energy efficiency problem of NOMA-based cloud radio access network (C-RANs), which maximized the energy efficiency by jointly optimizing power allocation, analog precoding, and digital precoding under the constraints of front-end link capacity and total transmit power. [PERSON] et al. [10] designed a cross-layer method suitable for NOMA systems to meet the delay requirements. By formulating the cross-layer scheduling problem as a joint optimization problem of transmission rate and decoding order, the optimal delay-power tradeoff was obtained in the NOMA system.