# Hybrid NOMA Protocol with Relay Adaptive AF/DF Collaboration and Its Modeling Analysis in NB-IoT
## 1 Introduction
To enable NB-IoT for more massive connections and lower energy consumption for cheap MTC devices, this paper considers the combination of NOMA and OMA, using NOMA technology to increase the number of connected users and the channel capacity on the one hand and using OMA technology to reduce the complexity of separating the information required by different users when the system only needs to retransmit for individual decoding failure and save overhead and reduce energy consumption on the other hand. Meanwhile, considering the wide coverage of NB-IoT, this paper considers a relay that can combine AF and DF to overcome the problem of excessive noise amplification caused by a single AF relay and the high complexity of decoding signals by a single DF relay, and thus improve the signal quality while expanding the coverage. Since NB-IoT is suitable for IoT scenarios with insensitive delay requirements, this paper is able to analyze the impact of multiple information retransmissions on system performance without considering the delay increase. Combined with the above discussion, this paper investigates the proposed Hybrid NOMA transmission protocol with an adaptive relay collaboration approach. Specifically, for the relay adaptive collaborative forwarding, during the system transmission, the transmitting node will decide whether to open the collaborative phase transmission and adaptively select the forwarding method, and the signal should be transmitted based on the positive answer (ACK) or negative answer (NACK) feedback from the device after the broadcast phase. If the channel conditions are good and the relay can decode the signal for collaborative transmission, the decoding and forwarding technique is selected, otherwise, the amplifying and forwarding technique is selected. For the hybrid multiple access mode, in order to avoid the resource waste situation of repeated decoding of the signal that has been correctly decoded in NOMA mode, the user can select the OMA mode to receive the signal. The main contributions of this paper are summarized as follows:  
* Unlike the old way of considering only one type of relay collaboration forwarding or simply using a single NOMA or OMA access, we combine the hybrid NOMA technology with AF/DF forwarding relay in NB-IoT, allowing users to flexibly switch between NOMA and OMA. The combination of NOMA and OMA with an adaptive selection of AF/DF forwarding allows users to flexibly switch between NOMA and OMA while enabling relay for cooperative transmission to improve the efficiency of relay usage, expand coverage and reduce energy consumption. * We analyze the performance of the Hybrid NOMA transmission model in NB-IoT based on the relay adaptive AF/DF collaboration approach and obtain closed-form expressions for the outage probability of each link. We establish a Markov model to compute the outage probability and throughput of the system. * In order to improve the reliability of network transmission, we study the case where the system can be retransmitted multiple times in the broadcast phase, comparing to most existing cases of Coordinated Direct and Relay Transmission (CDRT) for NOMA,where the system is allowed to retransmit only once. The numerical results prove that the system that allows multiple retransmissions is more suitable for mMTC devices with medium data rate and medium response time requirements in NB-IoT. The rest of this paper is distributed as follows. Section 2 introduces the transmission model and transmission protocol proposed in this paper. Sections 3 and 4 are the performance analysis of the system, establishing a Markov model to obtain the outage probability and throughput expressions. Section 5 analyzes the effect of the number of retransmissions on the system performance. Section 6 is the numerical simulation and analysis section.