  License-assisted access (LAA) is a promising technology to offload
dramatically increasing cellular traffic to unlicensed bands. Challenges arise
from the provision of quality-of-service (QoS) and the quantification of
capacity, due to the distributed and heterogeneous nature of LAA and legacy
systems (such as WiFi) coexisting in the bands. In this paper, we develop new
theories of the effective capacity to measure LAA under statistical QoS
requirements. A new four-state semi-Markovian model is developed to capture
transmission collisions, random backoffs, and lossy wireless channels of LAA in
distributed heterogeneous network environments. A closed-form expression for
the effective capacity is derived to comprehensively analyze LAA. The
four-state model is further abstracted to an insightful two-state equivalent
which reveals the concavity of the effective capacity in terms of transmit
rate. Validated by simulations, the concavity is exploited to maximize the
effective capacity and effective energy efficiency of LAA, and provide
significant improvements of 62.7% and 171.4%, respectively, over existing
approaches. Our results are of practical value to holistic designs and
deployments of LAA systems.
