  The use of unlicensed spectrum for future LTE systems raises concerns about
its impact on co-located Wi-Fi networks. We report herein on extensions to a
popular network simulator, known as ns-3, to model such coexistence. To our
knowledge, this simulator is novel in two important ways. First, it is
presently the only freely available simulator for coexistence studies, and as
open source software, it offers new opportunities for reproducible research and
collaborative model development. Second, we are not aware of other simulators
with similar wireless models that allow full system studies to be conducted,
such as performance evaluation of upper-layer protocols in a coexistence
environment. We illustrate the value of a system simulator by describing a
simulation campaign for the indoor scenario designed by 3GPP RAN1 using new
ns-3 models for Release 13 LTE Listen Before Talk (LBT) techniques, intended
for fair coexistence with Wi-Fi. LBT models have been widely simulated at the
link and physical layer by many companies in 3GPP RAN1. We first show that we
are able to tune the scenario to reproduce similar performance metrics to those
reported in the literature. We then extend the simulations by adding TCP to the
file transfers and explore the different behaviour observed. Our initial study
reveals that there remain many open questions about how LBT should be
implemented to achieve good coexistence properties, and that a broader scope of
implementation issues, traffic models, and fuller system models should be
studied further before drawing final conclusions on the LBT coexistence
performance.
