No abstract

Non-orthogonal multiple access (NOMA), as the newest member of the multiple access family, is envisioned to be an essential component of 5G mobile networks. The combination of NOMA and multiantenna multi-input multi-output (MIMO) technologies exhibits a significant potential in improving spectral efficiency and providing better wireless services to more users. In this article, we introduce the basic concepts of MIMO-NOMA and summarize the key technical problems in MIMO-NOMA systems. Then, we explore the problem formulation, beamforming, user clustering, and power allocation of single/multi-cluster MIMO-NOMA in the literature along with their limitations. Furthermore, we point out an important issue of the stability of successive interference cancellation (SIC) that arises using achievable rates as performance metrics in practical NOMA/MIMO-NOMA systems. Finally, we discuss incorporating NOMA with massive/millimeter wave MIMO, and identify the main challenges and possible future research directions in this area.

In this paper, we investigate resource allocation design for unmanned aerial vehicle (UAV)-enabled communication systems, where a UAV is dispatched to provide communications to multiple user nodes. Our objective is to maximize the communication system throughput by jointly optimizing the subcarrier allocation policy and the trajectory of the UAV, while taking into account the minimum required data rate for each user node, no-fly zones (NFZs), the maximum UAV cruising speed, and initial/final UAV locations. The design is formulated as a mixed integer non-convex optimization problem which is generally intractable. Subsequently, a computationallyefficient iterative algorithm is proposed to obtain a locally optimal solution. Simulation results illustrate that the performance of the proposed iterative algorithm approaches closely to that of the system without NFZ. In addition, the proposed algorithm can achieve a significant throughput gain compared to various benchmark schemes.

In this paper, we study the resource allocation and trajectory design for secure unmanned aerial vehicle (UAV)-enabled communication systems, where multiple multi-purpose UAV base stations are dispatched to provide secure communications to multiple legitimate ground users (GUs) in the existence of multiple eavesdroppers (Eves). Specifically, by leveraging orthogonal frequency division multiple access (OFDMA), active UAV base stations can communicate to their desired ground users via the assigned subcarriers while idle UAV base stations can serve as jammer simultaneously for communication security provisioning. To achieve fairness in secure communication, we maximize the average minimum secrecy rate per user by jointly optimizing the communication/jamming subcarrier allocation policy and the trajectory of UAVs, while taking into account the constraints on the minimum safety distance among multiple UAVs, the maximum cruising speed, the initial/final locations, and the existence of cylindrical no-fly zones (NFZs). The design is formulated as a mixed integer non-convex optimization problem which is generally intractable. Subsequently, a computationally-efficient iterative algorithm is proposed to obtain a suboptimal solution. Simulation results illustrate that the performance of the proposed iterative algorithm can significantly improve the average minimum secrecy rate compared to various baseline schemes. R. Li and J. An are with the secure UAV communication systems with a single-UAV was studied in [8], [14]-[16] with different system settings. However, due to the stringent requirements on UAV's size, weight, and power (SWAP), the performance achieved by deploying a single-UAV is still limited [6]. To achieve a higher efficiency in secure communications, multi-UAV cooperation was adopted in [17]-[20]. In particular, a jammer UAV can fly close to a potential eavesdropper based on demand by leveraging its mobility and opportunistically transmits artificial noise signal deliberately to combat the eavesdropping channels [17]. To improve the system security performance, [18] and [19] presented a cooperative jamming approach to safeguard the UAV's communication by exploiting artificial jamming transmission from other friend UAVs in the existence of a single-eavesdropper. With the consideration of fairness in two-UAV secure communications, [20] investigated the joint power allocation and trajectory design for the maximization of the minimum secrecy rate per user when one UAV is dispatched to convey confidential messages to a ground user where another cooperative UAV transmits a jamming signal. However, in [17]-[20], the role of the UAV is fixed where a communication/jamming UAV can only provide either communication/jamming signal during the whole time horizon.In contrast, a multi-purpose UAV, which can dynamically serve as a communication UAV or a jamming UAV, provides a high flexibility in trajectory design for secure UAV communications. March 17, 2020 DRAFT 3 For instance, when a communication UAV flies closer to an eave...