Abstract
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.
Highlights
Fifth generation (5G) cellular networks have ignited numerous research areas since its introduction
Excessively increasing nb lowers GN. This is because the excessive increase in blocklength does not result in further improvement in the decoding error; on the contrary, it increases the frame duration, leading to a decrease in the number of effective and non-redundant data bits transmitted per time unit by each IoT user equipments (UEs)
Various tradeoffs between the different network metrics and parameters have been highlighted to shed light on the importance of carefully selecting the number of packet replicas and blocklength. Both reliability and network GoodPut experience a peak by varying the number of packet replicas
Summary
Fifth generation (5G) cellular networks have ignited numerous research areas since its introduction. From a mathematical point of view, the moment generating function (MGF) is used to derive the joint probability density function (PDF) of the UEs’ transmit power and intra-cluster interference power, based on order statistics [27], [28] This facilitates the analytical derivation of network performance metrics, such as average packet latency, reliability, and GoodPut. For the average packet latency, both waiting time in a UE’s buffer and transmission time are considered. To the best of our knowledge, almost all the studies in the area of performance evaluation of NOMA-based transmissions are based on assumption that all the UEs in the network have saturated traffic, which implies that in each transmission frame, all the UEs are active and have data packets for transmission Another common assumption is that the number of active UEs in the whole network within each cluster is fixed and known, and that the order of SIC decoding is known to BS.
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