# Hybrid NOMA Protocol with Relay Adaptive AF/DF Collaboration and Its Modeling Analysis in NB-IoT
## 1 Introduction  
With the continuous development of IoT, a large number of IoT applications, such as remote control, health detection, smart meter, autonomous driving, and smart agriculture, are gradually entering people's production and life. To meet the massive connectivity needs of the IoT while providing a better service experience for IoT users, Machine-Type Communication (MTC) has attracted much attention from IoT scenarios. It refers to communication with terminals or devices that exchange data without human intervention. Unlike the existing human-type communication (HTC), the MTC model is characterized by periodic or infrequent small data transmissions that originate mainly from end devices [1]. In 2015, the International Telecommunication Union (ITU) defined enhanced mobile broadband (eMBB), massive machine-type communication (mMTC), and ultra-reliable low latency communication (URLLC) [2] as three classical application scenarios for 5 th generation mobile networks (5G), where mMTC and URLLC are considered as two typical MTCs. mMTC, unlike URLLC, is characterized by high connection density with a large number of low-power, low-cost devices, low end-to-end latency requirements, and the ability to transmit small data packets. As a new scenario extended by 5G, mMTC faces IoT services mainly to solve the problem that large-scale IoT connectivity cannot be realized in traditional communication. Narrowband IoT (NB-IoT) is a key technology for 5G mMTC and can be operated in the 5G NR band, similar to the current LTE [3]. Narrowband IoT has a very low bandwidthand can be implemented in the in-band, protected band, and independent operators of cellular networks. Therefore, it has a wide usable range, large coverage area, and low power consumption. All these should be implemented on the foundation of ensuring the security of information technology, which is one of the popular directions of current research. At present, related scholars have developed various algorithms based on algorithms to improve the security of the system in various types of environments [4; 5]. For large-scale links, currently, NB-IoT uses the orthogonal multiple access (OMA) scheme at 180 kHz bandwidth, which allows each subcarrier to serve only a single user, and thus the scheme cannot meet the demand for multi-connectivity requirements [6], and the application of non-orthogonal multiple access (NOMA) effectively solves this problem [7]. The key idea of NOMA technology is that multiple users can be served by the same radio resource simultaneously. In Power-Domain NOMA, the transmitter and receiver, respectively, use Superposition Coding (SC) and Serial Interference Cancellation (SIC) to improve spectrum efficiency and enable large-scale transmission. To further improve network coverage and reliability and meanwhile ensure user quality of service, hybrid NOMA [8; 9; 10; 11; 12], in which NOMA and OMA can be freely switched, and collaborative NOMA have been studied in the existing works of literature. The authors in [8] use the hybrid NOMA model to minimize the transmission delay of MIMO-mobile edge computing (MEC) in 6G communication networks. The advantageous performance of hybrid NOMA in cognitive radio systems is studied in [9]. Ref. [10] proposes a novel buffer-assisted relay selection scheme by fusing the concepts of NOMA and OMA. In [11], a hybrid NOMA/OMA-based relay selection scheme with buffer assistance in IoT is proposed. In [12], a hybrid NOMA scheme is considered for uplink wireless transmission systems, and an energy efficiency (EE) maximization problem is formulated. The collaborative communication technique [13] is a technique that uses relay nodes to increase user capacity, combat fading, and reduce the probability of system outages.