# Hybrid Cooperative Spectrum Sharing Protocols Based on OMA and NOMA in Cognitive Radio Networks
###### Abstract
Therefore, it is worth considering the opportunistic access spectrum of SU under the premise of providing a new protocol to guarantee PU transmission. In the overlay cognitive NOMA networks, the SU acts as a relay to assist transmission of the PU, and as a reward, the SU can send its own signal by using NOMA technology. A hybrid satellite-terrestrial spectrum sharing networks was established in [16]. Secondary networks can maintain stable operation when the outage probability of the system is guaranteed, thus exploring advantages of spectrum sharing opportunities. Similar to [16], the authors also investigate spectrum sharing protocols for hybrid satellite-terrestrial networks in [17]. To improve cooperative spectrum sharing, the NOMA power allocation profile is determined by channel conditioning, thus improving the fairness of spectrum allocation. In addition, the NOMA-based amplify-and-forward cooperative spectrum sharing systems, with imperfect channel state information and continuous interference cancellation, is proposed in [18]. The system performance was significantly improved by using NOMA relay assistance, which demonstrates the effectiveness of the NOMA-based amplify-and-forward cooperation scheme. Based on the above inspiration, what if a new collaborative spectrum sharing protocol was constructed by combining overlay NOMA and underlay OMA strategies? In this paper, we consider a hybrid cooperative cognitive network based on OMA and NOMA, which is composed of primary user (PU) pairs and secondary user (SU) pairs. PUs consists of a primary transmitter (PT) and primary receiver (PR). SUs consists of a secondary transmitter (ST) and a secondary receiver (SR). It is assumed that the PU's packet is transmitted at most two times while the SU's packet is allowed to transmit only once. Automatic repeat request (ARQ) technology is adopted at the PR node. When PT sends a packet, PR replies to acknowledgement (ACK) indicating correct receipt; otherwise, the PR replies to negative acknowledgement (NACK) indicating failure to receive the packets. The packet is discarded if PR sends NACK feedback after PT/ST retransmitting the packet previously failed. In this paper, transmission is divided into non-cognitive radio (non-CR) mode, underlay mode, and overlay cooperative mode. In non-CR mode, PT sends its packet by accessing the spectrum alone, while ST monitors the PU transmission and remains silent. If ACK feedback from PR, as a reward, transmission switch to underlay OMA mode. PT transmits a new packet within the threshold interference range tolerated by PUs, while the SU also sends its own packet; if PR sends NACK, PT retransmits a previously failed packet and ST remains silent. Otherwise, the packet is lost. If PR sends NACK feedback in the non-CR mode and ST has a copy of the PU's packet, transmission switch to overlay NOMA mode. ST uses NOMA technology to transmit PU's packet and its own packet. Otherwise, PT retransmits a previously failed packet. For another cooperative protocol proposed, orthogonal multiple access (OMA) is adopted for data transmission in the overlay mode. When PT sends a packet and PR sends NACK feedback, ST only acts as a relay to assist the PUs in transmitting previously failed packets. Otherwise, PT retransmits previously failed packets. The other two transmission modes are similar for hybrid cooperative CR networks based on OMA and NOMA. Finally, the comparison and analysis of the proposed two protocols show that the cooperative cognitive radio network based on NOMA and OMA is superior to the traditional cooperative cognitive radio network based on OMA. The main contributions are summarized as follows:  
* We propose a hybrid cooperative cognitive radio networks based on OMA and NOMA and compare with the proposed hybrid cooperative cognitive radio networks base on pure OMA.