  In this paper, we investigate unmanned aerial vehicle (UAV) assisted
communication systems that require quasi-balanced data rates in uplink (UL) and
downlink (DL), as well as users' heterogeneous traffic. To the best of our
knowledge, this is the first work to explicitly investigate joint UL-DL
optimization for UAV assisted systems under heterogeneous requirements. A
hybrid-mode multiple access (HMMA) scheme is proposed toward heterogeneous
traffic, where non-orthogonal multiple access (NOMA) targets high average data
rate, while orthogonal multiple access (OMA) aims to meet users' instantaneous
rate demands by compensating for their rates. HMMA enables a higher degree of
freedom in multiple access and achieves a superior minimum average rate among
users than the UAV assisted NOMA or OMA schemes. Under HMMA, a joint UL-DL
resource allocation algorithm is proposed with a closed-form optimal solution
for UL/DL power allocation to achieve quasi-balanced average rates for UL and
DL. Furthermore, considering the error propagation in successive interference
cancellation (SIC) of NOMA, an enhanced-HMMA scheme is proposed, which
demonstrates high robustness against SIC error and a higher minimum average
rate than the HMMA scheme.
