Internet-of-Things (IoT) networks have recently emerged to provide massive connectivity for many application scenarios and services. Additionally, developing spectrum-access strategies for a large number of nodes with sporadic data traffic behaviors in IoT networks has attracted much attention recently. However, developing such strategies becomes more challenging when ultra-reliable low-latency (URLL) transmissions are required. As IoT networks entail spectrum-efficient transmission schemes, non-orthogonal multiple-access (NOMA) has emerged as a key enabler for such networks. On the other hand, grant-free random-access (RA) techniques are particularly promising for high spectral-efficiency and massive connectivity, since they reduce signaling overhead, and packet latency. Therefore, in this paper, uplink RA-NOMA IoT networks with clustered IoT devices is studied, where short packet and diversity transmissions are adopted to meet the URLL requirements. To reduce the negative effect of diversity transmission on packet latency, multiple replicas of packets are accommodated within different resource blocks (RBs) in the same transmission time interval (TTI). The analytical expressions of network metrics, namely, average packet latency, reliability, and GoodPut are derived. Furthermore, the effect of the number of packet replicas, blocklength, and cluster size on the network metrics is evaluated. Finally, the analytical derivations are utilized to find the optimal values for the number of packet replicas, blocklength, and power control parameters, such that the network GoodPut is maximized, subject to URLL constraints.INDEX TERMS Internet-of-Things, low-latency, NOMA, random-access, ultra-reliability.
I. INTRODUCTIONFifth generation (5G) cellular networks have ignited numerous research areas since its introduction. The fundamental difference between 5G and the previous generations is that 5G is the driver for implementing two generic types of communications, namely, ultra-reliable low-latency communication (URLLC) and massive machine-type communication (mMTC) [1]. The combination of these two communication types along with the explosive and exponential growth in the number of smart devices, applications, and services is the advent of massive Internet-of-Things (mIoT) networks. This is in addition to the extensive emerging mission-critical applications and use cases, such as tactile Internet (involving remote motion control, tele-surgery, etc.), factory automation, Industrial IoT (IIoT), and those under the The associate editor coordinating the review of this manuscript and approving it for publication was Noor Zaman .
