  To meet the performance and complexity requirements from practical deployment
of non-orthogonal multiple access (NOMA) systems, several users are grouped
together for NOMA transmission while orthogonal resources are allocated among
groups. User grouping strategies have significant impact on the power
consumption and system performance. However, existing related studies divide
users into groups based on channel conditions, where diverse quality of service
(QoS) and interference have not been considered. In this paper, we focus on the
interference-aware user grouping strategy in NOMA systems, aiming at minimizing
power consumption with QoS constraints. We define a power consumption and
externality (PCE) function for each user to represent the power consumption
involved by this user to satisfy its QoS requirement as well as interference
that this user brings to others in the same group. Then, we extend the
definition of PCE to multi-user scenarios and convert the user grouping problem
into the problem of searching for specific negative loops in the graph.
Bellman-Ford algorithm is extended to find these negative loops. Furthermore, a
greedy suboptimal algorithm is proposed to approach the solution within
polynomial time. Simulation results show that the proposed algorithms can
considerably reduce the total power consumption compared with existing
strategies.
