70 research outputs found
Multiple Access in Aerial Networks: From Orthogonal and Non-Orthogonal to Rate-Splitting
Recently, interest on the utilization of unmanned aerial vehicles (UAVs) has
aroused. Specifically, UAVs can be used in cellular networks as aerial users
for delivery, surveillance, rescue search, or as an aerial base station (aBS)
for communication with ground users in remote uncovered areas or in dense
environments requiring prompt high capacity. Aiming to satisfy the high
requirements of wireless aerial networks, several multiple access techniques
have been investigated. In particular, space-division multiple access(SDMA) and
power-domain non-orthogonal multiple access (NOMA) present promising
multiplexing gains for aerial downlink and uplink. Nevertheless, these gains
are limited as they depend on the conditions of the environment. Hence, a
generalized scheme has been recently proposed, called rate-splitting multiple
access (RSMA), which is capable of achieving better spectral efficiency gains
compared to SDMA and NOMA. In this paper, we present a comprehensive survey of
key multiple access technologies adopted for aerial networks, where aBSs are
deployed to serve ground users. Since there have been only sporadic results
reported on the use of RSMA in aerial systems, we aim to extend the discussion
on this topic by modelling and analyzing the weighted sum-rate performance of a
two-user downlink network served by an RSMA-based aBS. Finally, related open
issues and future research directions are exposed.Comment: 16 pages, 6 figures, submitted to IEEE Journa
Enhanced User Grouping and Power Allocation for Hybrid mmWave MIMO-NOMA Systems
Non-orthogonal multiple access (NOMA) and millimeter wave (mmWave) are two key enabling technologies for the fifth-generation (5G) mobile networks and beyond. In this paper, we consider uplink communications with a hybrid beamforming structure and focus on improving the spectral efficiency (SE) and energy efficiency (EE) of mmWave multiple-input multiple-output (MIMO)-NOMA systems with enhanced user grouping and power allocation. It is noted that the optimization of the SE/EE is a challenging task due to the non-linear programming nature of the corresponding problem involving user grouping, beam selection, and power allocation. Our idea is to decompose the overall optimization problem into a mixed integer problem comprised of user grouping and beam selection only, followed by a continuous problem involving power allocation and digital beamforming design. Exploiting the directionality property of mmWave channels, we first propose a novel initial agglomerative nesting (AGNES) based user grouping algorithm by taking advantage of the channel correlations. To avoid the prohibitively high complexity of the brute-force search approach and to address the overlapping beam problem, we propose two suboptimal low-complexity user grouping and beam selection schemes, the two-stage direct AGNES (D-AGNES) scheme and the joint successive AGNES (S-AGNES) scheme. We also introduce the quadratic transform (QT) to recast the non-convex power allocation optimization problem into a convex one subject to a minimum required data rate of each user. The continuous problem is solved by iteratively optimizing the power and the digital beamforming. Extensive simulation results have shown that our proposed mmWave-NOMA design outperforms the conventional orthogonal multiple access (OMA) scenario and the state-of-art NOMA schemes
Non-Orthogonal Multiple Access For Near-Field Communications
The novel concept of near-field non-orthogonal multiple access (NF-NOMA)
communications is proposed. The near-filed beamfocusing enables NOMA to be
carried out in both angular and distance domains. Two novel frameworks are
proposed, namely, single-location-beamfocusing NF-NOMA (SLB-NF-NOMA) and
multiple-location-beamfocusing NF-NOMA (MLB-NF-NOMA). 1) For SLB-NF-NOMA, two
NOMA users in the same angular direction with distinct quality of service (QoS)
requirements can be grouped into one cluster. The hybrid beamformer design and
power allocation problem is formulated to maximize the sum rate of the users
with higher QoS (H-QoS) requirements. To solve this problem, the analog
beamformer is first designed to focus the energy on the H-QoS users and the
zero-forcing (ZF) digital beamformer is employed. Then, the optimal power
allocation is obtained. 2) For MLB-NF-NOMA, the two NOMA users in the same
cluster can have different angular directions. The analog beamformer is first
designed to focus the energy on both two NOMA users. Then, a singular value
decomposition (SVD) based ZF (SVD-ZF) digital beamformer is designed.
Furthermore, a novel antenna allocation algorithm is proposed. Finally, a
suboptimal power allocation algorithm is proposed. Numerical results
demonstrate that the NF-NOMA can achieve a higher spectral efficiency and
provide a higher flexibility than conventional far-field NOMA
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