# Hybrid NOMA Protocol with Relay Adaptive AF/DF Collaboration and Its Modeling Analysis in NB-IoT
## 1 Introduction
Collaborative relay technology was first proposed by [PERSON] and other scholars in 2003 and was demonstrated that collaborative transmission can obtain higher data rates and better cope with channel variations under the use of an unrestricted system model and the use of fixed transmission scheme system models, respectively [14; 15]. Then two basic types of relaying collaboration methods, i.e., amplify-and-forward (AF) and decode-and-forward (DF) were proposed by some scholars. With the continuous development of collaborative communication technology, collaborative communication techniques have been used in real communication scenarios and have been included in standards such as IEEE 802.11s-WLAN, 802.16j-WMAN, and 802.20-MBWA [16]. For long-range users whose channel conditions are poor and no direct link can be established between the base station and the user, the relay can help to deliver the information of such users to extend the propagation range. The literature [17] provides an early research scenario on combining NOMA with collaborative communication, which includes a source node with multiple destination nodes, and concludes that collaborative NOMA outperforms ordinary NOMA. In [18], an energy harvesting relay-assisted NOMA NB-IoT network is considered and an online optimization algorithm with deep reinforcement learning is used to improve the proportional fairness and total throughput among NB-IoT devices. Unlike the above-mentioned literature where the transmitting node retransmits the packet at most once, based on the outage performance of NOMA system with HARQ, [19] obtains the tradeoff between the number of retransmissions of superposition signal and the power distribution of the system. [PERSON] [20] designs the system in which each user can perform one retransmission if the previous packet is not successfully decoded and obtains the false packet rate and throughput in a collaborative scenario by using a Markov model. Refs. [21; 22] use the collaboration technique of buffer-assisted relay to help the H-NOMA system reduce outage probability and increase throughput. Although a series of previous hybrid NOMA models allow nodes to switch modes flexibly to better adapt to the different requirements generated by different environments on the receiver side and get better system performance, the above-mentioned studies modelsin collaborative communication still have limitations in using only a single relay forwarding approach. For example, when the decode-and-forward (DF) relay incorrectly decodes the signal or fails to receive the signal, the relay is unable to complete the task of collaborative transmission, which is not only underutilized but also consumes unnecessary energy. In contrast, although the operation of the Amplify-and-Forward (AF) relay is simple, it also amplifies the noise while amplifying the signal required by the destination node, which does not necessarily obtain a good performance when the channel state is poor. We note that in [23; 24; 25], the authors consider AF and DF together to form a model in which the relay can adaptively select the collaboration method. Considering the integration of the respective advantages and disadvantages of AF and DF collaboration approaches, the literature [23] proposes a threshold-based adaptive relay collaboration approach, which makes the relay a better collaboration performance. In [24], a hybrid AF/DF cooperative system is formed to improve the achievable end-to-end performance. The source node in the literature [25] communicates with a pair of energy harvesting (EH) user devices through a multi-antenna relay node to finally obtain a new expression for the secrecy rate.