Near-Field LoS/NLoS Channel Estimation for RIS-Aided MU-MIMO Systems: Piece-Wise Low-Rank Approximation Approach

We study the channel estimation problem for a reconfigurable intelligent surface (RIS)-assisted millimeter-wave (mmWave) multi-user multiple-input multiple-output (MU-MIMO) system. In particular, it is assumed that the channel between a RIS and a base station (BS) exhibits a near-field line-of-sight (LoS) channel, which is a dominant signal path in mmWave communication systems. Due to the high-rankness and non-sparsity of the RIS-BS channel matrix in our system, the state-of-the-art (SOTA) methods, which are constructed based on far-field or near-field non-LoS (NLoS) channel, cannot provide attractive estimation performances. We for the first time propose an efficient near-field LoS/NLoS channel estimation method for RIS-assisted MU-MIMO systems by means of a piece-wise low-rank approximation. Specifically, an effective channel (to be estimated) is partitioned into piece-wise effective channels containing low-rank structures and then, they are estimated via collaborative low-rank approximation. The proposed method is named PW-CLRA. Via simulations, we verify the effectiveness of the proposed PW-CLRA.

Asymptotically Near-Optimal Hybrid Beamforming for mmWave IRS-Aided MIMO Systems

Hybrid beamforming is an emerging technology for massive multiple-input multiple-output (MIMO) systems due to the advantages of lower complexity, cost, and power consumption. Recently, intelligent reflection surface (IRS) has been proposed as the cost-effective technique for robust millimeter-wave (mmWave) MIMO systems. Thus, it is required to jointly optimize a reflection vector and hybrid beamforming matrices for IRS-aided mmWave MIMO systems. Due to the lack of RF chain in the IRS, it is unavailable to acquire the TX-IRS and IRS-RX channels separately. Instead, there are efficient methods to estimate the so-called effective (or cascaded) channel in literature. We for the first time derive the near-optimal solution of the aforementioned joint optimization only using the effective channel. Based on our theoretical analysis, we develop the practical reflection vector and hybrid beamforming matrices by projecting the asymptotic solution into the modulus constraint. Via simulations, it is demonstrated that the proposed construction can outperform the state-of-the-art (SOTA) method, where the latter even requires the knowledge of the TX-IRS ad IRS-RX channels separately. Furthermore, our construction can provide the robustness for channel estimation errors, which is inevitable for practical massive MIMO systems.

Channel Estimation for Reconfigurable Intelligent Surface Aided mmWave MU-MIMO Systems : Hybrid Receiver Architectures

Channel estimation is one of the key challenges for the deployment of reconfigurable intelligence surface (RIS)-aided communication systems. In this paper, we study the channel estimation problem of RIS-aided mmWave multi-user multiple-input multiple-output (MU-MIMO) systems especially when a hybrid receiver architecture is adopted. For this system, we propose a simple yet efficient channel estimation method using the fact that cascaded channels (to be estimated) have low-dimensional common column space. In the proposed method, the reflection vectors at the RIS and the RF combining matrices at the BS are designed such that the training observations are suitable for estimating the common column space and the user-specific coefficient matrices via a collaborative low-rank approximation. Via simulations, we demonstrate the effectiveness of the proposed channel estimation method compared with the state-of-the-art ones.

On Practical Robust Reinforcement Learning: Practical Uncertainty Set and Double-Agent Algorithm

We study a robust reinforcement learning (RL) with model uncertainty. Given nominal Markov decision process (N-MDP) that generate samples for training, an uncertainty set is defined, which contains some perturbed MDPs from N-MDP for the purpose of reflecting potential mismatched between training (i.e., N-MDP) and testing environments. The objective of robust RL is to learn a robust policy that optimizes the worst-case performance over an uncertainty set. In this paper, we propose a new uncertainty set containing more realistic MDPs than the existing ones. For this uncertainty set, we present a robust RL algorithm (named ARQ-Learning) for tabular case and characterize its finite-time error bound. Also, it is proved that ARQ-Learning converges as fast as Q-Learning and the state-of-the-art robust Q-Learning while ensuring better robustness to real-world applications. Next, we propose {\em pessimistic} agent that efficiently tackles the key bottleneck for the extension of ARQ-Learning into the case with larger or continuous state spaces. Incorporating the idea of pessimistic agents into the famous RL algorithms such as Q-Learning, deep-Q network (DQN), and deep deterministic policy gradient (DDPG), we present PRQ-Learning, PR-DQN, and PR-DDPG, respectively. Noticeably, the proposed idea can be immediately applied to other model-free RL algorithms (e.g., soft actor critic). Via experiments, we demonstrate the superiority of our algorithms on various RL applications with model uncertainty.

Low-Complexity Symbol-Level Precoding for MU-MISO Downlink Systems with QAM Signals

This study proposes the construction of a transmit signal for large-scale antenna systems with cost-effective 1-bit digital-to-analog converters in the downlink. Under quadrature-amplitude-modulation constellations, it is still an open problem to overcome a severe error floor problem caused by its nature property. To this end, we first present a feasibility condition which guarantees that each user's noiseless signal is placed in the desired decision region. For robustness to additive noise, we formulate an optimization problem, we then transform the feasibility conditions to cascaded matrix form. We propose a low-complexity algorithm to generate a 1-bit transmit signal based on the proposed optimization problem formulated as a well-defined mixed-integer-linear-programming. Numerical results validate the superiority of the proposed method in terms of detection performance and computational complexity.

Distributed Online Learning with Multiple Kernels

We consider the problem of learning a nonlinear function over a network of learners in a fully decentralized fashion. Online learning is additionally assumed, where every learner receives continuous streaming data locally. This learning model is called a fully distributed online learning (or a fully decentralized online federated learning). For this model, we propose a novel learning framework with multiple kernels, which is named DOMKL. The proposed DOMKL is devised by harnessing the principles of an online alternating direction method of multipliers and a distributed Hedge algorithm. We theoretically prove that DOMKL over T time slots can achieve an optimal sublinear regret, implying that every learner in the network can learn a common function which has a diminishing gap from the best function in hindsight. Our analysis also reveals that DOMKL yields the same asymptotic performance of the state-of-the-art centralized approach while keeping local data at edge learners. Via numerical tests with real datasets, we demonstrate the effectiveness of the proposed DOMKL on various online regression and time-series prediction tasks.

Multiple Kernel-Based Online Federated Learning

Online federated learning (OFL) becomes an emerging learning framework, in which edge nodes perform online learning with continuous streaming local data and a server constructs a global model from the aggregated local models. Online multiple kernel learning (OMKL), using a preselected set of P kernels, can be a good candidate for OFL framework as it has provided an outstanding performance with a low-complexity and scalability. Yet, an naive extension of OMKL into OFL framework suffers from a heavy communication overhead that grows linearly with P. In this paper, we propose a novel multiple kernel-based OFL (MK-OFL) as a non-trivial extension of OMKL, which yields the same performance of the naive extension with 1/P communication overhead reduction. We theoretically prove that MK-OFL achieves the optimal sublinear regret bound when compared with the best function in hindsight. Finally, we provide the numerical tests of our approach on real-world datasets, which suggests its practicality.

Pool-based sequential active learning with multi kernels

We study a pool-based sequential active learning (AL), in which one sample is queried at each time from a large pool of unlabeled data according to a selection criterion. For this framework, we propose two selection criteria, named expected-kernel-discrepancy (EKD) and expected-kernel-loss (EKL), by leveraging the particular structure of multiple kernel learning (MKL). Also, it is identified that the proposed EKD and EKL successfully generalize the concepts of popular query-by-committee (QBC) and expected-model-change (EMC), respectively. Via experimental results with real-data sets, we verify the effectiveness of the proposed criteria compared with the existing methods.

Active Learning with Multiple Kernels

Online multiple kernel learning (OMKL) has provided an attractive performance in nonlinear function learning tasks. Leveraging a random feature approximation, the major drawback of OMKL, known as the curse of dimensionality, has been recently alleviated. In this paper, we introduce a new research problem, termed (stream-based) active multiple kernel learning (AMKL), in which a learner is allowed to label selected data from an oracle according to a selection criterion. This is necessary in many real-world applications as acquiring true labels is costly or time-consuming. We prove that AMKL achieves an optimal sublinear regret, implying that the proposed selection criterion indeed avoids unuseful label-requests. Furthermore, we propose AMKL with an adaptive kernel selection (AMKL-AKS) in which irrelevant kernels can be excluded from a kernel dictionary 'on the fly'. This approach can improve the efficiency of active learning as well as the accuracy of a function approximation. Via numerical tests with various real datasets, it is demonstrated that AMKL-AKS yields a similar or better performance than the best-known OMKL, with a smaller number of labeled data.