  This article investigates the prominent dilemma between capacity and
reliability in heterogeneous ultra-dense distributed networks, and advocates a
new measure of effective capacity to quantify the maximum sustainable data rate
of a link while preserving the quality-of-service (QoS) of the link in such
networks. Recent breakthroughs are brought forth in developing the theory of
the effective capacity in heterogeneous ultra-dense distributed networks.
Potential applications of the effective capacity are demonstrated on the
admission control, power control and resource allocation of such networks, with
substantial gains revealed over existing technologies. This new measure is of
particular interest to ultra-dense deployment of the emerging fifth-generation
(5G) wireless networks in the unlicensed spectrum, leveraging the capacity gain
brought by the use of the unlicensed band and the stringent reliability
sustained by 5G in future heterogeneous network environments.
