83 research outputs found
Beamfocusing Optimization for Near-Field Wideband Multi-User Communications
A near-field wideband communication system is studied, wherein a base station
(BS) employs an extremely large-scale antenna array (ELAA) to serve multiple
users situated within its near-field region. To facilitate the near-field
beamfocusing and mitigate the wideband beam split, true-time delayer
(TTD)-based hybrid beamforming architectures are employed at the BS. Apart from
the fully-connected TTD-based architecture, a new sub-connected TTD-based
architecture is proposed for enhancing energy efficiency. Three wideband
beamfocusing optimization approaches are proposed to maximize spectral
efficiency for both architectures. 1) Fully-digital approximation (FDA)
approach: In this approach, the TTD-based hybrid beamformers are optimized to
approximate the optimal fully-digital beamformers using block coordinate
descent. 2) Penalty-based FDA approach: In this approach, the penalty method is
leveraged in the FDA approach to guarantee the convergence to a stationary
point of the spectral maximization problem. 3) Heuristic two-stage (HTS)
approach: In this approach, the closed-form TTD-based analog beamformers are
first designed based on the outcomes of near-field beam training and the
piecewise-near-field approximation. Subsequently, the low-dimensional digital
beamformer is optimized using knowledge of the low-dimensional equivalent
channels, resulting in reduced computational complexity and channel estimation
complexity. Our numerical results unveil that 1) the proposed approaches
effectively eliminate the near-field beam split effect, and 2) compared to the
fully-connected architecture, the proposed sub-connected architecture exhibits
higher energy efficiency and imposes fewer hardware limitations on TTDs and
system bandwidth.Comment: 30 pages, 11 figure
Hybrid Beamforming With Sub-arrayed MIMO Radar: Enabling Joint Sensing and Communication at mmWave Band
In this paper, we propose a beamforming design for dual-functional
radar-communication (DFRC) systems at the millimeter wave (mmWave) band, where
hybrid beamforming and sub-arrayed MIMO radar techniques are jointly exploited.
We assume that a base station (BS) is serving a user equipment (UE) located in
a Non-Line-of-Sight (NLoS) channel, which in the meantime actively detects
multiple targets located in a Line-of-Sight (LoS) channel. Given the optimal
communication beamformer and the desired radar beampattern, we propose to
design the analog and digital beamformers under non-convex constant-modulus
(CM) and power constraints, such that the weighted summation of the
communication and radar beamforming errors is minimized. The formulated
optimization problem can be decomposed into three subproblems, and is solved by
the alternating minimization approach. Numerical simulations verify the
feasibility of the proposed beamforming design, and show that our approach
offers a favorable performance tradeoff between sensing and communication.Comment: 5 pages, 2 figures, submitted to ICASSP 201
General Rank Multiuser Downlink Beamforming With Shaping Constraints Using Real-valued OSTBC
In this paper we consider optimal multiuser downlink beamforming in the
presence of a massive number of arbitrary quadratic shaping constraints. We
combine beamforming with full-rate high dimensional real-valued orthogonal
space time block coding (OSTBC) to increase the number of beamforming weight
vectors and associated degrees of freedom in the beamformer design. The
original multi-constraint beamforming problem is converted into a convex
optimization problem using semidefinite relaxation (SDR) which can be solved
efficiently. In contrast to conventional (rank-one) beamforming approaches in
which an optimal beamforming solution can be obtained only when the SDR
solution (after rank reduction) exhibits the rank-one property, in our approach
optimality is guaranteed when a rank of eight is not exceeded. We show that our
approach can incorporate up to 79 additional shaping constraints for which an
optimal beamforming solution is guaranteed as compared to a maximum of two
additional constraints that bound the conventional rank-one downlink
beamforming designs. Simulation results demonstrate the flexibility of our
proposed beamformer design
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