Uplink Cooperative NOMA for Cellular-Connected UAV

Abstract

Aerial-ground interference mitigation is a challenging issue in the\ncellular-connected unmanned aerial vehicle (UAV) communications. Due to the\nstrong line-of-sight (LoS) air-to-ground (A2G) channels, the UAV may\nimpose/suffer more severe uplink/downlink interference to/from the cellular\nbase stations (BSs) than the ground users. To tackle this challenge, we propose\nto apply the non-orthogonal multiple access (NOMA) technique to the uplink\ncommunication from a UAV to cellular BSs, under spectrum sharing with the\nexisting ground users. However, for our considered system, traditional NOMA\nwith local interference cancellation (IC), termed non-cooperative NOMA, may\nprovide very limited gain compared to the OMA. This is because there are many\nco-channel BSs due to the LoS A2G channels and thus the UAV's rate performance\nis severely limited by the BS with the worst channel condition with the UAV. To\nimprove the UAV's achievable rate, a new cooperative NOMA scheme is proposed by\nexploiting the backhaul links among BSs. Specifically, some BSs with better\nchannel conditions are selected to decode the UAV's signals first, and then\nforward the decoded signals to their backhaul-connected BSs for IC. To\ninvestigate the optimal design of cooperative NOMA, we maximize the weighted\nsum-rate of the UAV and ground users by jointly optimizing the UAV's rate and\npower allocations over multiple resource blocks. However, this problem is hard\nto be solved optimally. To obtain useful insights, we first consider two\nspecial cases with egoistic and altruistic transmission strategies of the UAV,\nrespectively, and solve their corresponding problems optimally. Next, we\nconsider the general case and propose an efficient suboptimal solution via the\nalternating optimization. Numerical results show that the proposed cooperative\nNOMA yields significant throughput gains than the OMA and the non-cooperative\nNOMA benchmark.\n