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In this paper, we investigate unmanned aerial vehicle (UAV) assistedcommunication systems that require quasi-balanced data rates in uplink (UL) anddownlink (DL), as well as users' heterogeneous traffic. To the best of ourknowledge, this is the first work to explicitly investigate joint UL-DLoptimization for UAV assisted systems under heterogeneous requirements. Ahybrid-mode multiple access (HMMA) scheme is proposed toward heterogeneoustraffic, where non-orthogonal multiple access (NOMA) targets high average datarate, while orthogonal multiple access (OMA) aims to meet users' instantaneousrate demands by compensating for their rates. HMMA enables a higher degree offreedom in multiple access and achieves a superior minimum average rate amongusers than the UAV assisted NOMA or OMA schemes. Under HMMA, a joint UL-DLresource allocation algorithm is proposed with a closed-form optimal solutionfor UL/DL power allocation to achieve quasi-balanced average rates for UL andDL. Furthermore, considering the error propagation in successive interferencecancellation (SIC) of NOMA, an enhanced-HMMA scheme is proposed, whichdemonstrates high robustness against SIC error and a higher minimum averagerate than the HMMA scheme.

Toward UL-DL Rate Balancing: Joint Resource Allocation and Hybrid-Mode Multiple Access for UAV-BS Assisted Communication Systems