Browsing by Author "Castedo, Luis"

Now showing 1 - 9 of 9
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    A rank-constrained coordinate ascent approach to hybrid precoding for the downlink of wideband massive MIMO systems
    (2023-02-01) González-Coma, José P.; Fresnedo, Óscar; Castedo, Luis
    An innovative approach to hybrid analog-digital precoding for the downlink of wideband massive MIMO systems is developed. The proposed solution, termed Rank-Constrained Coordinate Ascent (RCCA), starts seeking the full-digital pre coder that maximizes the achievable sum-rate over all the frequency subcarriers while constraining the rank of the overall transmit covariance matrix. The frequency-flat constraint on the analog part of the hybrid precoder and the non-convex nature of the rank constraint are circumvented by transforming the original problem into a more suitable one, where a convenient structure for the transmit covariance matrix is imposed. Such structure makes the resulting full-digital precoder particularly adequate for its posterior analog-digital factorization. An addi tional problem formulation to determine an appropriate power allocation policy according to the rank constraint is also pro vided. The numerical results show that the proposed method outperforms baseline solutions even for practical scenarios with high spatial diversity.
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    Alternating Minimization for Wideband Multiuser IRS-aided MIMO Systems under Imperfect CSI
    (IEEE, 2023-11) Pérez-Adán, Darian; Joham, Michael; Fresnedo, Óscar; González-Coma, José P.; Castedo, Luis; Utschick, Wolfgang
    This work focuses on wideband intelligent reflecting surface (IRS)-aided multiuser MIMO systems. One of the major challenges of this scenario is the joint design of the frequency dependent base station (BS) precoder and user filters, and the IRS phase-shift matrix which is frequency flat and common to all the users. In addition, we consider that the channel state information (CSI) is imperfect at both the transmitter and the receivers. A statistical model for the imperfect CSI is developed and exploited for the system design. A minimum mean square error (MMSE) approach is followed to determine the IRS phase-shift matrix, the transmit precoders, and the receiving filters. The broadcast (BC)-multiple access channel (MAC) duality is used to solve the optimization problem following an alternating minimization approach. Numerical results show that the proposed approach leads to substantial performance gains with respect to baseline strategies that neglect the inter-user interference and do not optimize the IRS phase-shift matrix. Further performance gains are obtained when incorporating into the system design the statistical information of the channel estimation errors.
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    Channel estimation and hybrid precoding for frequency selective multiuser mmWave MIMO systems
    (IEEE, 2018-02) González-Coma, José P.; González-Prelcic, Nuria; Castedo, Luis; Heath, Robert W. Jr.
    Configuring the hybrid precoders and combiners in a millimeter wave (mmWave) multiuser (MU) multiple-input multiple-output (MIMO) system is challenging in frequency selective channels. In this paper, we develop a system that uses compressive estimation on the uplink to configure precoders and combiners for the downlink (DL). In the first step, the base station (BS) simultaneously estimates the channels from all the mobile stations (MSs) on each subcarrier. To reduce the number of measurements required, compressed sensing techniques are developed that exploit common support on the different subcarriers. In the second step, exploiting reciprocity and the channel estimates, the base station designs hybrid precoders and combiners. Two algorithms are developed for this purpose, with different performance and complexity tradeoffs: 1) a factorization of the purely digital solution, and 2) an iterative hybrid design. Extensive numerical experiments evaluate the proposed solutions comparing to state-of-the-art strategies, and illustrating design tradeoffs in overhead, complexity, and performance.
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    Design of Linear Precoders for Correlated Sources in MIMO Multiple Access Channels
    (IEEE, 2018) Suárez-Pascal, Pedro; González-Coma, José P.; Fresnedo, Óscar; Castedo, Luis
    This work focuses on distributed linear precod ing when users transmit correlated information over a fading Multiple-Input and Multiple-Output Multiple Access Channel. Precoders are optimized in order to minimize the sum-Mean Square Error (MSE) between the source and the estimated symbols. When sources are correlated, minimizing the sum-MSE results in a non-convex optimization problem. Precoders for an arbitrary number of users and transmit and receive antennas are thus obtained via a projected steepest-descent algorithm and a low-complexity heuristic approach. For the more restrictive case of two single-antenna users, a closed-form expression for the minimum sum-MSE precoders is derived. Moreover, for the scenario with a single receive antenna and any number of users, a solution is obtained by means of a semidefinite relaxation. Finally, we also consider precoding schemes where the precoders are decomposed into complex scalars and unit norm vectors. Simulation results show a significant improvement when source correlation is exploited at precoding, especially for low SNRs and when the number of receive antennas is lower than the number of transmitting nodes.
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    Hybrid LISA for Wideband Multiuser Millimeter Wave Communication Systems under Beam Squint
    (IEEE, 2019) González-Coma, José P.; Utschick, Wolfgang; Castedo, Luis
    This work jointly addresses user scheduling and precoder/combiner design in the downlink of a wideband mil limeter wave (mmWave) communications system. We consider Orthogonal frequency-division multiplexing (OFDM) modulation to overcome channel frequency selectivity and obtain a number of equivalent narrowband channels. Hence, the main challenge is that the analog preprocessing network is frequency flat and common to all the users at the transmitter side. Moreover, the effect of the signal bandwidth over the Uniform Linear Array (ULA) steering vectors has to be taken into account to design the hybrid precoders and combiners. The proposed algorithmic solution is based on Linear Successive Allocation (LISA), which greedily allocates streams to different users and computes the corresponding precoders and combiners. By taking into account the rank limitations imposed by the hardware at transmission and reception, the performance loss in terms of achievable sum rate for the hybrid approach is negligible. Numerical experiments show that the proposed method exhibits excellent performance with reasonable computational complexity.
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    Hybrid Precoding with Time-Modulated Arrays for mmWave MIMO Systems
    (IEEE, 2018) González-Coma, José P.; Maneiro-Catoira, Roberto; Castedo, Luis
    We consider the utilization of Time-Modulated Arrays (TMAs) as a simple and cost-effective approach to hybrid digital-analog precoding in millimeter wave (mmWave) Multiple-Input Multiple-Output (MIMO) systems. Instead of conventional Variable Phase Shifters (VPSs), our proposed TMA hybrid precoders use Radio Frequency (RF) switches followed by 1-bit VPSs in the analog part. We study the insertion losses at mmWave of a TMA Analog Precoding Network (APN) in terms of Sideband Radiation (SR) and hardware efficiency. In addition, we present different algorithms for the design of both the baseband and RF parts of a TMA hybrid precoder. The proposed methods exhibit different trade-offs between performance, complexity, and power efficiency. Finally, TMA precoders are compared to those implemented with conventional VPSs in terms of insertion losses, chip area and cost, concluding that precoding with TMAs is a competitive solution for mmWave MIMO systems.
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    Low-Complexity Distance-Based Scheduling for Multi-User XL-MIMO Systems
    (IEEE, 2021) González-Coma, José P.; López-Martínez, F.J.; Castedo, Luis
    We introduce Distance-Based Scheduling (DBS), a new technique for user selection in downlink multi-user commu nications with extra-large (XL) antenna arrays. DBS categorizes users according to their equivalent distance to the antenna array. Such categorization effectively accounts for inter-user interfer ence while largely reducing the computational burden. Results show that (i) DBS achieves the same performance as the reference zero-forcing beamforming scheme with a lower complexity; (ii) a simplified version of DBS achieves a similar performance when realistic spherical-wavefront (SW) propagation features are considered; (iii) SW propagation brings additional degrees of freedom, which allows for increasing the number of served users.
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    Power Efficient Scheduling and Hybrid Precoding for Time Modulated Arrays
    (IEEE, 2020) González-Coma, José P.; Castedo, Luis
    We consider power efficient scheduling and precoding solutions for multiantenna hybrid digital-analog transmission systems that use Time-Modulated Arrays (TMAs) in the analog domain. TMAs perform beamforming with switches instead of conventional Phase Shifters (PSs). The extremely low insertion losses of switches, together with their reduced power consumption and cost make TMAs attractive in emerging technologies like massive Multiple-Input Multiple-Output (MIMO) and millimeter wave (mmWave) systems. We propose a novel analog processing network based on TMAs and provide an angular scheduling algorithm that overcomes the limitations of conventional approaches. Next, we pose a convex optimization problem to determine the analog precoder. This formulation allows us to account for the Sideband Radiation (SR) effect inherent to TMAs, and achieve remarkable power efficiencies with a very low impact on performance. Computer experiments results show that the proposed design, while presenting a significantly better power efficiency, achieves a throughput similar to that obtained with other strategies based on angular selection for conventional architectures.
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    Wideband Hybrid Precoding using Time Modulated Arrays
    (IEEE, 2020) González-Coma, José P.; Castedo, Luis
    Hybrid digital-analog precoding is a cost effective solution for transmitting over the large bandwidthsand huge antenna arrays available in the millimeter wave frequency bands. In this work we focus on the transmission of wideband signals over hybrid precoders that utilize Time-Modulated Arrays in the analog domain. We consider analog precoders constructed with Single-Pole-Double-Throw switches which are both flexible and efficient. We pose two Orthogonal Frequency-Division Multiplexing symbol configurations to cope with the harmonic interference introduced by the Time-Modulated Array. One does not take advantage of the wireless channel frequency diversity and the other does. We optimize digital and analog precoders to maximize the achievable rate in both symbol configurations. Optimization takes into account the switching devices efficiency and the inherent losses of this antenna technique. Finally, we show the excellent performance obtained with the proposed approach, in terms of achievable rate, compared to that of conventional phased arrays.