  In a heterogeneous cellular network (HetNet), consider that a base station in
the HetNet is able to simultaneously schedule and serve K users in the downlink
by performing the power-domain non-orthogonal multiple access (NOMA) scheme.
This paper aims at the preliminary study on the downlink coverage and
throughput performances of the HetNet with the non-cooperative and the
(proposed) cooperative NOMA schemes. First, the coverage probability and link
throughput of K users in each cell are studied and their accurate expressions
are derived for the non-cooperative NOMA scheme in which no BSs are coordinated
to jointly transmit the NOMA signals for a particular user. We show that the
coverage and link throughput can be largely reduced if transmit power
allocations among the K users do not satisfy the constraint derived. Next, we
analyze the coverage and link throughput of K users for the cooperative NOMA
scheme in which the void BSs without users are coordinated to enhance the
farthest NOMA user in a cell. The derived accurate results show that
cooperative NOMA can significantly improve the coverage and link throughput of
all users. Finally, we show that there exist optimal power allocation schemes
that maximize the average cell coverage and throughput under some derived power
allocation constraints and numerical results validate our analytical findings.
