  Unmanned aerial vehicle (UAV) assisted cellular communication is gaining
significant interest recently. Although it offers several advantages over
terrestrial communication, UAV communication suffers from two main
shortcomings. The typical untethered UAV (uUAV) has a limited battery power
supply and therefore limited flying time, and it needs an extra wireless
backhaul link to connect users to the core network. In this paper, we propose
the utilization of the tethered UAV (tUAV) to assist the cellular network,
where the tether provides power supply and connects the tUAV to the core
network through high capacity link. The tUAV however has a limited mobility due
to the limited tether length. A stochastic geometry-based analysis is provided
for the coverage probability of an UAV-assisted cellular network where the
mobile users located within a circular hot-spot. For that setup, we analyze and
compare two scenarios: (i) utilizing uUAV and (ii) utilizing tUAV, for
offloading the terrestrial base station (TBS). We capture the aforementioned
limitations of each of the uUAV and the tUAV in our analysis. A novel user
association analysis is provided given the TBS and the UAV locations. Next, we
study the optimal locations of the uUAV and the tUAV to maximize the coverage
probability. Multiple useful insights are revealed. For instance, numerical
results show that tUAVs outperform uUAVs when the tether length is above 75 m,
given that the uUAV is available for 80% of the time due to its battery
limitations.
