  In this paper, we consider unmanned aerial vehicles (UAVs) assisted cellular
communication system, where UAVs can be used as amplify-and-forward (AF)
relays. The effective channel with UAV assisted communication can be modeled as
a Rayleigh product-channel, and we derive a tight lower-bound of the ergodic
capacity in closed-form. With the obtained lower-bound, trade-offs between the
transmit power and the equipped number of antennas of the UAVs can be analyzed.
Alternatively, for a giving setting of users and the base-transceiver station
(BTS), the needed transmit power and number of antennas for the UAVs can be
derived in order to have a higher ergodic capacity with the UAV assisted
communication than that without it.
