arXiv subjects

Kabuto Arai

Publications and source records attributed to Kabuto Arai.

5 recordsLinked to original sources

arXiv2026

Revisiting Beamforming Design for Stable Millimeter-Wave Communications Under Blockages

This paper investigates robust analog beamforming for millimeter-wave (mmWave) communications under stochastic path blockages using multi-panel arrays. Conventional designs concentrate beams on the line-of-sight (LoS) path to maximize array gain, but this approach is highly vulnerable to sudden disconnections when the LoS path is blocked. To overcome this limitation, we propose a multi-beam design that exploits both LoS and non-line-of-sight (NLoS) paths for stable communications. The major contribution of this work is to establish a theoretical foundation for multi-beam design, where closed-form expressions for the cumulative distribution function (CDF) and outage probability of the spectral efficiency (SE) are derived. To design the optimal multi-beam based on the derived outage probability, we formulate a panel allocation problem to determine the assignment of panels to specific paths. The optimization problem can be solved by two algorithms based on brute-force search. Through computer simulations, the validity of the theoretical analysis for multi-beam design is confirmed, and the proposed algorithms substantially reduce the outage probability while maintaining average SE performance, thereby achieving stable communication.

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arXiv2025

Expectation Propagation-Based Signal Detection for Highly Correlated MIMO Systems

Large-scale multiple-input-multiple-output (MIMO) systems typically operate in dense array deployments with limited scattering environments, leading to highly correlated and ill-conditioned channel matrices that severely degrade the performance of message-passing-based detectors. To tackle this issue, this paper proposes an expectation propagation (EP)-based detector, termed overlapping block partitioning EP (OvEP). In OvEP, the large-scale measurement vector is partitioned into partially overlapping blocks. For each block and its overlapping part, a low-complexity linear minimum mean square error (LMMSE)-based filter is designed according to the partitioned structure. The resulting LMMSE outputs are then combined to generate the input to the denoiser. In this combining process, subtracting the overlapping-part outputs from the block outputs effectively mitigates the adverse effects of inter-block correlation induced by high spatial correlation. The proposed algorithm is consistently derived within the EP framework, and its fixed point is theoretically proven to coincide with the stationary point of a relaxed Kullback- Leibler (KL) minimization problem. The mechanisms underlying the theoretically predicted performance improvement are further clarified through numerical simulations. The proposed algorithm achieves performance close to conventional LMMSE-EP with lower computational complexity.

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arXiv2025

Joint Pilot Allocation and Sequence Design for MIMO-OFDM Systems With Channel Sparsity

This paper proposes a joint optimization of pilot subcarrier allocation and non-orthogonal sequence for multiple-input-multiple-output (MIMO)-orthogonal frequency-division multiplexing (OFDM) systems under compressed sensing (CS)-based channel estimation exploiting delay and angle sparsity. Since the performance of CS-based approaches depends on a coherence metric of the sensing matrix in the measurement process, we formulate a joint optimization problem to minimize this coherence. Due to the discrete nature of subcarrier allocation, a straightforward formulation of the joint optimization results in a mixed-integer nonlinear program (MINLP), which is computationally intractable due to the combinatorial explosion of allocation candidates. To overcome the intractability of discrete variables, we introduce a block sparse penalty for pilots across all subcarriers, which ensures that the power of some unnecessary pilots approaches zero. This framework enables joint optimization using only continuous variables. In addition, we propose an efficient computation method for the coherence metric by exploiting the structure of the sensing matrix, which allows its gradient to be derived in closed form, making the joint optimization problem solvable in an efficient way via a gradient descent approach. Numerical results confirm that the proposed pilot sequence exhibits superior coherence properties and enhances the CS-based channel estimation performance.

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arXiv2025

Joint Channel and Data Estimation for Multiuser Extremely Large-Scale MIMO Systems

This paper proposes a joint channel and data estimation (JCDE) algorithm for uplink multiuser extremely large-scale multiple-input-multiple-output (XL-MIMO) systems. The initial channel estimation is formulated as a sparse reconstruction problem based on the angle and distance sparsity under the near-field propagation condition. This problem is solved using non-orthogonal pilots through an efficient low complexity two-stage compressed sensing algorithm. Furthermore, the initial channel estimates are refined by employing a JCDE framework driven by both non-orthogonal pilots and estimated data. The JCDE problem is solved by sequential expectation propagation (EP) algorithms, where the channel and data are alternately updated in an iterative manner. In the channel estimation phase, integrating Bayesian inference with a model-based deterministic approach provides precise estimations to effectively exploit the near-field characteristics in the beam-domain. In the data estimation phase, a linear minimum mean square error (LMMSE)-based filter is designed at each sub-array to address the correlation due to energy leakage in the beam-domain arising from the near-field effects. Numerical simulations reveal that the proposed initial channel estimation and JCDE algorithm outperforms the state-of-the-art approaches in terms of channel estimation, data detection, and computational complexity.

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arXiv2025

Channel Estimation for Hybrid MIMO Systems With Array Model Errors and Beam Squint Effects

This paper proposes a channel estimation method for hybrid wideband multiple-input-multiple-output (MIMO) systems in high-frequency bands, including millimeter-wave (mmWave) and sub-terahertz (sub-THz), in the presence of beam squint effects and array errors arising from hardware impairments and environmental time fluctuations such as thermal effects and dynamic motion of the array. Although conventional channel estimation methods calibrate array errors through offline operation with large training pilots, the calibration errors remain due to time-varying array errors. Therefore, the proposed channel estimation method calibrates array errors online with small pilot overhead. In the proposed method, array response matrices are explicitly decomposed into a small number of physical parameters including path gains, angles and array errors, which are iteratively estimated by alternating optimization based on a maximum likelihood (ML) criterion. To enhance the convergence performance, we introduce a switching mechanism from an ongrid algorithm to an off-gird algorithm depending on the estimation accuracy of the array error during algorithmic iterations. Furthermore, we introduce an approximate mutual coupling model to reduce the number of parameters. The reduction of parameters not only lowers computational complexity but also mitigates overfitting to noisy observations. Numerical simulations demonstrate that the proposed method effectively works online even with small pilot overhead in the presence of array errors.

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