  With the exponential increase in mobile users, the mobile data demand has
grown tremendously. To meet these demands, cellular operators are constantly
innovating to enhance the capacity of cellular systems. Consequently, operators
have been reusing the licensed spectrum spatially, by deploying 4G/LTE small
cells (e.g., Femto Cells) in the past. However, despite the use of small cells,
licensed spectrum will be unable to meet the consistently rising data traffic
because of data-intensive applications such as augmented reality or virtual
reality (AR/VR) and on-the-go high-definition video streaming. Applications
such AR/VR and online gaming not only place extreme data demands on the
network, but are also latency-critical. To meet the QoS guarantees, cellular
operators have begun leveraging the unlicensed spectrum by coexisting with
Wi-Fi in the 5 GHz band. The standardizing body 3GPP, has prescribed cellular
standards for fair unlicensed coexistence with Wi-Fi, namely LTE Licensed
Assisted Access (LAA), New Radio in unlicensed (NR-U), and NR in Millimeter.
The rapid roll-out of LAA deployments in developed nations like the US, offers
an opportunity to study and analyze the performance of unlicensed coexistence
networks through real-world ground truth. Thus, this paper presents a
high-level overview of past, present, and future of the research in small cell
and unlicensed coexistence communication technologies. It outlines the vision
for future research work in the recently allocated unlicensed spectrum: The 6
GHz band, where the latest Wi-Fi standard, IEEE 802.11ax, will coexist with the
latest cellular technology, 5G New Radio (NR) in unlicensed.
