Capacity Analysis of Downlink NOMA-Based Coexistent HTC/MTC in UDN

The coexistence of human-type communications (HTC) users and machine-type communications (MTC) devices is inevitable in the next generation of cellular communications. In this paper, we study the impact of the association schemes on the downlink capacity of ultra-dense networks (UDNs). We adopt non-orthogonal multiple access (NOMA) where HTC users and MTC devices share the spectrum based on power-NOMA approach. The capacity in terms of average rate and area spectral efficiency (ASE) is analyzed for two extreme association schemes, namely, connect to active (C2A) and connect to closest (C2C). Furthermore, we investigate the network performance while moving from one extreme to the other. On one hand, C2A is an efficient scheme from HTC users' perspective where it can keep most of the existing base stations (BSs) in the idle mode; saving energy and reducing interference. However, it provides unsatisfactory performance for the MTC devices. On the other hand, moving towards C2C is favorable to MTC devices, however, it deteriorates the ASE of the HTC users and increases the transmission power density. Accordingly, we seek a compromise between these two contradicting schemes. Accurate expressions for the network performance are derived using tools from stochastic geometry and validated by Monte Carlo simulations.