  In this paper, we present a stochastic geometry-based analysis for the
coverage probability of multi-tier heterogeneous networks employing
interference coordination for non-orthogonal multiple access (NOMA). Two basic
and conventional interference coordination alternatives; strict fractional
frequency reuse (FFR) and soft frequency reuse (SFR) are analyzed, and their
performance gains on different instances are illustrated. Coverage probability
analysis has been extended to that of spectral efficiency for a fair
performance evaluation between different FFR methods. The analysis covers both
open-access networks with micro- and pico-cells, and closed-access networks
with femto-cells. For heterogeneous networks, NOMA is observed to achieve
better spectral efficiency than OFDMA, while OFDMA provides better coverage.
Both analysis and numerical results reveal that strict FFR provides better
coverage while SFR provides spectral efficiency advantage in open-access.
However, strict FFR outperforms SFR for both measures in closed-access
networks.
