No abstract

In this paper, an easy analysis framework is developed to evaluate the reliability and security of the secondary receivers (SU-Rxs) in millimeter wave (mmWave) non-orthogonal multiple access (NOMA) aided cognitive radio (CR) networks, where secondary transmitters (SU-Txs) transmit confidential messages to SU-Rxs belonging to different regions under the interference temperature constraint of the primary receiver (PU-Rx). Considering that randomly located eavesdroppers and primary user may be in the signal beam emitted by the SU-Txs, information leakage and interference to the primary network will be inevitable issues. Motivated by these challenges, combining the key features of multi-cell mmWave networks and NOMA communications, the closed-form expressions of three key performance metrics of connection outage probability (COP), secrecy outage probability (SOP), and secrecy throughput (ST) are derived by using the sector eavesdropper-exclusion zone, and the reliability and secrecy performance of the secondary networks are measured by different system parameters. The obtained performance evaluation results have demonstrated that the interference temperature constraint of the PU-Rx can be used to balance the security and reliability of the secondary networks. The reliability and security of paired NOMA users can be improved by using reasonable power allocation factor and sector eavesdropper-exclusion zone. Meanwhile, the connection performance of paired NOMA users is better than orthogonal multiple access (OMA). Furthermore, various system parameters can be reasonably set in conjunction with blockage and inter-cell interference to achieve optimal network's performance. INDEX TERMS Non-orthogonal multiple access (NOMA), millimeter wave (mmWave), cognitive radio (CR), stochastic geometry.

Non-orthogonal multiple access (NOMA) is expected as a promising technology for satisfying massive connectivity requirements in Internet of Things (IoT). Besides, short-packet communications are a notable characteristic in IoT for low delay transmissions or massive machine-type communications (MTC). The co-channel interference from NOMA and packet error caused by finite packet will reduce system performance. In view of this, this paper adopts automatic repeat request (ARQ) to assist shortpacket communications for NOMA networks where a near user (NU) and a far user (FU) are paired to share the same non-orthogonal communication resources. The closed-form expressions for average packet error rate (APER) and effective throughput (ET) of both NU and FU are derived over Nakagami-m channels. For gaining an insight into system performance, the performance of the scheme without ARQ is also studied. Results show that ARQ improves APER for both NU and FU, and an optimal power allocation factor or packet length exists for achieving higher ET. INDEX TERMS Automatic repeat request, short-packet communications, non-orthogonal multiple access.

Considering the coverage and short-packet communication requirements, this paper proposes a hybrid automatic repeat request (HARQ) assisted short-packet communications for cooperative Internet of Things (IoT) networks, where the signals are transmitted through short-packets. Since the short-packet communications will bring a higher packet error rate than long-packet communications, HARQ is adopted to improve transmission reliability by allowing retransmit the signals which cannot be detected correctly. For lighting the burden of the transmitter, the retransmitted signals are only sent from the relay. According to the detection capability of the user, selection combining (SC) scheme and maximum ratio combining (MRC) scheme are designed to detect the multiple transmitted signals. Besides, a set of closed-form expressions of the average packet error rate (APER) and effective throughput (ET) are derived over Nakagami-m channels in the proposed SC and MRC schemes. For providing more insights into system performance, the asymptotic results are also provided. Simulation results demonstrate that HARQ will significantly improve reliability and transmission efficiency. Also, the MRC scheme outperforms the SC scheme in terms of APER and ET metrics, however, when the transmit power at the relay is high enough or the packet length is big enough, they achieve nearly the same performance. INDEX TERMS Hybrid automatic repeat request, short-packet communications, cooperative Internet of Things.