  The emerging edge caching provides an effective way to reduce service delay
for mobile users. However, due to high deployment cost of edge hosts, a
practical problem is how to achieve minimum delay under a proper edge
deployment strategy. In this letter, we provide an analytical framework for
delay optimal mobile edge deployment in a partially connected wireless network,
where the request files can be cached at the edge hosts and cooperatively
transmitted through multiple base stations. In order to deal with the
heterogeneous transmission requirements, we separate the entire transmission
into backhaul and wireless phases, and propose average user normalized delivery
time (AUNDT) as the performance metric. On top of that, we characterize the
trade-off relations between the proposed AUNDT and other network deployment
parameters. Using the proposed analytical framework, we are able to provide the
optimal mobile edge deployment strategy in terms of AUNDT, which provides a
useful guideline for future mobile edge deployment.
