  This paper proposes a novel approach to improve the performance of a
heterogeneous network (HetNet) supported by dual connectivity (DC) by adopting
multiple unmanned aerial vehicles (UAVs) as passive relays that carry
reconfigurable intelligent surfaces (RISs). More specifically, RISs are
deployed under the UAVs termed as UAVs-RISs that operate over the micro-wave
($\mu$W) channel in the sky to sustain a strong line-of-sight (LoS) connection
with the ground users. The macro-cell operates over the $\mu$W channel based on
orthogonal multiple access (OMA), while small base stations (SBSs) operate over
the millimeter-wave (mmW) channel based on non-orthogonal multiple access
(NOMA). We study the problem of total transmit power minimization by jointly
optimizing the trajectory/velocity of each UAV, RISs' phase shifts, subcarrier
allocations, and active beamformers at each BS. The underlying problem is
highly non-convex and the global optimal solution is intractable. To handle it,
we decompose the original problem into two subproblems, i.e., a subproblem
which deals with the UAVs' trajectories/velocities, RISs' phase shifts, and
subcarrier allocations for $\mu$W; and a subproblem for active beamforming
design and subcarrier allocation for mmW. In particular, we solve the first
subproblem via the dueling deep Q-Network (DQN) learning approach by developing
a distributed algorithm which leads to a better policy evaluation. Then, we
solve the active beamforming design and subcarrier allocation for the mmW via
the successive convex approximation (SCA) method. Simulation results exhibit
the effectiveness of the proposed resource allocation scheme compared to other
baseline schemes. In particular, it is revealed that by deploying UAVs-RISs,
the transmit power can be reduced by 6 dBm while maintaining similar guaranteed
QoS.
