  Ultra-dense networks are widely regarded as a promising solution to
explosively growing applications of Internet-of-Things (IoT) mobile devices
(IMDs). However, complicated and severe interferences need to be tackled
properly in such networks. To this end, both orthogonal multiple access (OMA)
and non-orthogonal multiple access (NOMA) are utilized at first. Then, in order
to attain a goal of green and secure computation offloading, under the
proportional allocation of computational resources and the constraints of
latency and security cost, joint device association, channel selection,
security service assignment, power control and computation offloading are done
for minimizing the overall energy consumed by all IMDs. It is noteworthy that
multi-step computation offloading is concentrated to balance the network loads
and utilize computing resources fully. Since the finally formulated problem is
in a nonlinear mixed-integer form, it may be very difficult to find its
closed-form solution. To solve it, an improved whale optimization algorithm
(IWOA) is designed. As for this algorithm, the convergence, computational
complexity and parallel implementation are analyzed in detail. Simulation
results show that the designed algorithm may achieve lower energy consumption
than other existing algorithms under the constraints of latency and security
cost.
