  Incorporating mobile edge computing (MEC) in the Internet of Things (IoT)
enables resource-limited IoT devices to offload their computation tasks to a
nearby edge server. In this paper, we investigate an IoT system assisted by the
MEC technique with its computation task subjected to sequential task
dependency, which is critical for video stream processing and other intelligent
applications. To minimize energy consumption per IoT device while limiting task
processing delay, task offloading strategy, communication resource, and
computation resource are optimized jointly under both slow and fast fading
channels. In slow fading channels, an optimization problem is formulated, which
is mixed-integer and non-convex. To solve this challenging problem, we
decompose it as a one-dimensional search of task offloading decision problem
and a non-convex optimization problem with task offloading decision given.
Through mathematical manipulations, the non-convex problem is transformed to be
a convex one, which is shown to be solvable only with the simple Golden search
method. In fast fading channels, optimal online policy depending on instant
channel state is derived. In addition, it is proved that the derived policy
will converge to the offline policy when channel coherence time is low, which
can help to save extra computation complexity. Numerical results verify the
correctness of our analysis and the effectiveness of our proposed strategies
over existing methods.
