# Hybrid NOMA Protocol with Relay Adaptive AF/DF Collaboration and Its Modeling Analysis in NB-IoT
## 2 System Model and Protocol Description  
Firstly, we introduce the system model in this paper. We will simplify and abstract the realistic communication model in some way. In this paper, we consider a single-cell communication scenario and study the system performance of this model under the transmission protocol to be proposed in this paper. Therefore, we will use a system model, as shown in Figure 1. In the model, we use the model consisting of an Evolved Node B (eNB), a Relay (R). Meanwhile, according to the ITU's definition of NB-IoT devices, we consider that two devices in the model, i.e., a URLLC and an mMTC (we call them user \(U_{1}\), user \(U_{2}\), respectively) randomly and uniformly distributed in the cell, and assume that there is no mutual interference between each cell. Each node is equipped with an antenna and operates in a half-duplex manner, and it is assumed that each node has perfect access to the local channel state information (CSI). In the collaboration phase, relay nodes can use AF or DF method to perform collaborative transmission depending on the acceptance of both devices, and the feedback channel is considered ideal when each packet can be feedback ACK/NACK immediately after the end of transmission and the feedback information can be received correctly. Both the direct transmission link and the collaborative link use mutually independent flat Rayleigh slow fading channel as the channel model, and the interference noise model uses an additive Gaussian white noise (AWGN) model. The distances of the links \(eNB\to U_{1}\), \(eNB\to R\), and \(R\to U_{i}\) are \(d_{i},d_{r},\delta_{i},i\in\{1,2\}\). The channel fading coefficients of each link are \(h_{i},h_{r},g_{i},i\in\{1,2\}\), which are complex Gaussian random variables with zero mean and variance \(\sigma^{2}=\frac{1}{\lambda}=d^{-m}\), where \((\lambda,d)\in\big{\{}({{{\lambda}_{h1}}},{{{d}_{1}}}),({{{\lambda}_{h2}}},{{ {d}_{2}}}),({{{\lambda}_{r}}},{{{d}_{r}}}),({{{\lambda}_{g1}}},{{{\xi}_{1}}}), ({{{\lambda}_{g2}}},{{{\xi}_{2}}})\big{\}}\), \(m\) is the path loss coefficient, so all random variables \({{{\left|{h}\right|}^{2}},h\in\{h_{1},h_{2},h_{r},g_{1},g_{2}\}}\) obey the exponential distribution, i.e., \({{\left|{h}\right|}^{2}}\sim e(\lambda)\). Based on the above model, we consider that NB-IoT, which has gradually become a key technology for 5G massive machine-type communication, can no longer meet the demand of connecting multiple devices if only the original orthogonal multiple access method is used, while the use of non-orthogonal multiple access method will increase the system overhead and impose higher equipment requirements due to the use of superposition coding technology and continuous interference cancellation technology. Therefore, this paper proposes a hybrid NOMA and OMA transmission protocol, which allows flexible switching between NOMA/OMA access methods during the collaboration phase. In order  
Figure 1: System model. to improve the coverage and avoid the inefficiency of relay collaboration caused by the monolithic relay collaboration approach, we allow the relay to choose AF or DF collaboration mode for collaborative transmission based on the hybrid NOMA mode according to its self-decoding situation. When the system is in the broadcast phase, the eNB will broadcast the superimposed signal to the relay and the two users based on NOMA, as shown in Figure 2a. When the system is in the collaboration phase, if only a single user fails to decode and needs collaborative transmission, the relay can transmit the required signal for the user based on OMA mode, which avoids the case of repeated decoding of the decoded signal of the correct user by using continuous interference cancellation under NOMA. Figure 2b,c show the collaborative transmission process of DF relay based on OMA for user 1 and user 2 respectively. If both users fail to decode, Figure 2d shows the process of collaborative transmission between two users by NOMA-based DF relay.