  In this paper, we jointly consider communication, caching and computation in
a multi-user cache-assisted mobile edge computing (MEC) system, consisting of
one base station (BS) of caching and computing capabilities and multiple users
with computation-intensive and latency-sensitive applications. We propose a
joint caching and offloading mechanism which involves task uploading and
executing for tasks with uncached computation results as well as computation
result downloading for all tasks at the BS, and efficiently utilizes multi-user
diversity and multicasting opportunities. Then, we formulate the average total
energy minimization problem subject to the caching and deadline constraints to
optimally allocate the storage resource at the BS for caching computation
results as well as the uploading and downloading time durations. The problem is
a challenging mixed discrete-continuous optimization problem. We show that
strong duality holds, and obtain an optimal solution using a dual method. To
reduce the computational complexity, we further propose a low-complexity
suboptimal solution. Finally, numerical results show that the proposed
suboptimal solution outperforms existing comparison schemes.
