Performance Evaluation of Uplink Grant-Free Access Networks Based on Spreading-Based NOMA

The pressing need to support connectivity to an ever-increasing number of devices has motivated the development of new channel access schemes, which achieve throughput gains with low channel access latency. In this article, we characterize the performance achieved by an uplink grant-free channel access scheme adopting a spreading-based nonorthogonal multiple access (NOMA) scheme. Modeling the network as a stochastic geometry problem and assuming flat fading, we characterize the distribution of the received power from each interferer, the aggregate interference, and the signal-to-interference-plus-noise ratio (SINR) for a given transmitter when Zadoff–Chu (ZC) sequences are adopted to spread the data in orthogonal frequency-division multiplexing (OFDM) symbols. The probabilities of false alarm and detection are derived and compared with simulation results, validating the accuracy of the proposed model. Finally, we provide results of the distribution of the number of successfully decoded transmissions depending on the number of competing nodes, the detection threshold, the networks’ density, and ZC sequence lengths, showing essential insights that can be taken into consideration to design new medium access control (MAC) schemes capable of regulating the uplink channel access of multiple nodes adopting spreading-based NOMA schemes.