  The performances of the fifth generation (5G) wireless communication systems
are significantly affected by edge cache and transport network. These emerging
components bring substantial costs of the placement and utilization, and the
evaluation of the cost impact is beyond the capability of traditional
performance metrics, including spectral efficiency (SE) and energy efficiency
(EE). In this article, economical energy efficiency (E3) is proposed, whose
core idea is to take SE/EE and cost into account to evaluate comprehensive
gains when different kinds of advanced technologies are used in 5G systems. The
E3results are shown when the transport network and edge cache are separately or
jointly used. Open issues in terms of modeling the cost, E3optimization based
radio resource allocation, and E3optimization for internet of things, are
identified as well.
