In non‐orthogonal multiple access (NOMA) systems, it may not be advisable to send the superimposed signal of all users to individual users with retransmission requirements during the retransmission phase. This paper investigates a novel cognitive radio network based on hybrid NOMA/orthogonal multiple access (CR‐H‐NOMA/OMA) in underlay spectrum sharing, whereby a hybrid NOMA/OMA cooperative mechanism is proposed for the secondary network by considering that the access modes of secondary users (SUs) and relays can be switched between NOMA and OMA. In order to compensate the loss of throughput caused by successive interference cancellation (SIC) during the cooperative phase, one or two relays with the best channel quality between themselves and the destination users are selected according to the states of the initial signal decoded by SUs, using truncated automatic repeat request (T‐ARQ) to retransmit only the desired signal of SU instead of the entire superimposed signal. Exact closed‐form expressions for outage probabilities of SUs and system throughput of the secondary network are derived under the interference constraint of the primary network to evaluate the performance of the proposed cooperative scheme. Numerical and simulation results demonstrate the superiority of the proposed hybrid NOMA/OMA cooperative scheme when compared with conventional pure NOMA and pure OMA based on time division multiple access (TDMA) in terms of secondary throughput. Also, the outage and throughput performance depend critically on the choices of key paraments such as power allocation factor and target rate.

In this paper, we consider a novel cooperative underlay cognitive radio network based on non-orthogonal multiple access (CR-NOMA) with adaptive relay selection and power allocation. In secondary networks, dedicated relay assistance and user assistance are used to achieve communication between the base station and the far (and near) user. Here, a two-stage adaptive relay selection and power allocation strategy is proposed to maximize the achievable data rate of the far user while ensuring the service quality of near user. Furthermore, the closed-form expressions of outage probability of two secondary users are derived, respectively, under interference power constraints, revealing the impact of transmit power, number of relays, interference threshold and target data rate on system outage probability. Numerical results and simulations validate the advantages of the established cooperation and show that the proposed adaptive relay selection and power allocation strategy has better outage performance.

This paper proposes a multifrequency seamless dual-link handover scheme based on beamforming for the high-speed railway (HSR) communication, i.e., when the train travels to the edge of overlapping area of the adjacent two cells, a gain beam is assigned to the overlapping area by the target base station (BS) using a beamforming technique to cover the entire region to enhance the received signal strength (RSS) and handover opportunity. The switching antenna is allowed to handover multiple times to improve the traditional scheme and reduce the handover failure probability (HFP) greatly. In the process of signal reception, considering the impact of the intercell cochannel interference on the RSS, the proposed scheme uses signal to interference ratio (SIR) instead of the RSS to indicate the received signal quality to optimize handover mode. The handover success probability (HSP) is analyzed to describe the relationship between train location and the probability. We also establish a probabilistic model and corresponding handover algorithm for the proposed scheme to complete the handover operations and theoretically analyze a series of indexes including handover trigger probability (HTP), HFP, communication interruption probability (CIP), and HSP. Theoretical and experimental results show that the proposed scheme can effectively improve the HTP and HSP and greatly reduce the HFP and the CIP.