# Hybrid Cooperative Spectrum Sharing Protocols Based on OMA and NOMA in Cognitive Radio Networks
###### Abstract  
In cognitive radio networks (CRNs), the performance of the primary users (PUs) may suffer adverse effects from the secondary users (SUs) if the spectrum of PUs is haphazardly shared with SUs. In this paper, we propose a hybrid cognitive cooperative protocol based on orthogonal multiple access (OMA) and non-orthogonal multiple access (NOMA) that improves spectrum utilization by allowing the SUs opportunistic access to the spectrum of the Pus, while guaranteeing the performance of the PU. Specifically, the system can switch between non-cognitive transmission mode, underlay OMA mode, and overlay OMA/NOMA mode, according to the automatic repeat request (ARQ) feedback of PU. The SU has the opportunity to acquire the spectrum to activate the underlay OMA and overlay OMA/NOMA modes only if it listens to the acknowledge (ACK) or negative acknowledge (NACK) feedback from the PU. In order to describe the switching between these three switching modes, a Markov model is developed to analyze the corresponding steady-state probabilities and end-to-end outage probabilities. So, we derive closed-form expressions for the throughput of PU and SU to investigate the spectrum utilization. Numerical and simulation results show that the proposed hybrid cooperative cognitive protocol outperforms the pure OMA hybrid cooperative cognitive protocol. c 2022122683 2022122683 2022122683 2022122683 20221222683 20221222683 202212  
In addition, integrating non-orthogonal multiple access (NOMA) into CR has shown the possibility to meet 5G requirements of high throughput, low latency, massive connectively and more spectrum sharing [4; 5]. In NOMA system, superimpose coding is adopted at the transmitter side to allow multiple users to simultaneously access the spectrum, and successive interference cancellation (SIC) [6; 7] is used at the receiver side to separate the signals of the different users. The most popular type of NOMA is the power domain NOMA, where the transmitter sends a superimposed signal to all receivers and serves them at different power factors according to their channel conditions. The method of combining NOMA and cognitive radio is called CR-NOMA. Currently, a wealth of research results has been achieved in CR-NOMA related to underlay NOMA and overlay NOMA. In the underlay cognitive NOMA network, the secondary transmitter (ST) can provide services for multiple secondary receivers (SR) with NOMA technology under the interference of the primary transmitter (PT). How to minimize the interference of SU on PU link [8; 9] is a problem worth pondering. A spectrum sharing protocol with a peak interference constrain was proposed in [10], the results show that the interference channel between the ST and the PU has a more severe effect on the average achievable rate. In [11], the authors use orthogonal spatial modulation technique to improve the spectrum efficiency of the PU and enhance the spectrum usage of the SU. However, none of the work mentioned above takes advantage of the new NOMA and OMA technologies. In recent years, there has also been a lot of work on cooperative spectrum sharing on NOMA technology [12; 13]. A novel NOMA assisted cooperative spectrum sharing network with full-duplex ST was presented in [12]. The PT needs the help from the ST to relay its information and as an incentive, ST can access the spectrum of the primary network and send both primary and secondary messages by using NOMA. Similar to the cooperation approach of [12], the authors propose a cooperative spectrum sharing protocol based on NOMA in [14]. The proposed protocol can achieve full-rate at the PU without degrading its outage performance. For better cooperative communication and spectrum sharing, CRNs based on automatic request repeat (ARQ) technology [15] have also been proposed. The throughput of the proposed AF relay and DF relay forwarding models is compared and analyzed. The authors conclude that the proposed protocol provides a more balanced scheme for PU and SU performance.