188 research outputs found
Downlink Achievable Rate Analysis for FDD Massive MIMO Systems
Multiple-Input Multiple-Output (MIMO) systems with large-scale transmit antenna arrays, often called massive MIMO, are a very promising direction for 5G due to their ability to increase capacity and enhance both spectrum and energy efficiency. To get the benefit of massive MIMO systems, accurate downlink channel state information at the transmitter (CSIT) is essential for downlink beamforming and resource allocation. Conventional approaches to obtain CSIT for FDD massive MIMO systems require downlink training and CSI feedback. However, such training will cause a large overhead for massive MIMO systems because of the large dimensionality of the channel matrix. In this dissertation, we improve the performance of FDD massive MIMO networks in terms of downlink training overhead reduction, by designing an efficient downlink beamforming method and developing a new algorithm to estimate the channel state information based on compressive sensing techniques. First, we design an efficient downlink beamforming method based on partial CSI. By exploiting the relationship between uplink direction of arrivals (DoAs) and downlink direction of departures (DoDs), we derive an expression for estimated downlink DoDs, which will be used for downlink beamforming. Second, By exploiting the sparsity structure of downlink channel matrix, we develop an algorithm that selects the best features from the measurement matrix to obtain efficient CSIT acquisition that can reduce the downlink training overhead compared with conventional LS/MMSE estimators. In both cases, we compare the performance of our proposed beamforming method with traditional methods in terms of downlink achievable rate and simulation results show that our proposed method outperform the traditional beamforming methods
Multi-user Linear Precoding for Multi-polarized Massive MIMO System under Imperfect CSIT
The space limitation and the channel acquisition prevent Massive MIMO from
being easily deployed in a practical setup. Motivated by current deployments of
LTE-Advanced, the use of multi-polarized antennas can be an efficient solution
to address the space constraint. Furthermore, the dual-structured precoding, in
which a preprocessing based on the spatial correlation and a subsequent linear
precoding based on the short-term channel state information at the transmitter
(CSIT) are concatenated, can reduce the feedback overhead efficiently. By
grouping and preprocessing spatially correlated mobile stations (MSs), the
dimension of the precoding signal space is reduced and the corresponding
short-term CSIT dimension is reduced. In this paper, to reduce the feedback
overhead further, we propose a dual-structured multi-user linear precoding, in
which the subgrouping method based on co-polarization is additionally applied
to the spatially grouped MSs in the preprocessing stage. Furthermore, under
imperfect CSIT, the proposed scheme is asymptotically analyzed based on random
matrix theory. By investigating the behavior of the asymptotic performance, we
also propose a new dual-structured precoding in which the precoding mode is
switched between two dual-structured precoding strategies with 1) the
preprocessing based only on the spatial correlation and 2) the preprocessing
based on both the spatial correlation and polarization. Finally, we extend it
to 3D dual-structured precoding.Comment: accepted to IEEE Transactions on Wireless Communication
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Secure Communication for Spatially Sparse Millimeter-Wave Massive MIMO Channels via Hybrid Precoding
In this paper, we investigate secure communication over sparse millimeter-wave (mm-Wave) massive multiple-input multiple-output (MIMO) channels by exploiting the spatial sparsity of legitimate user's channel. We propose a secure communication scheme in which information data is precoded onto dominant angle components of the sparse channel through a limited number of radio-frequency (RF) chains, while artificial noise (AN) is broadcast over the remaining nondominant angles interfering only with the eavesdropper with a high probability. It is shown that the channel sparsity plays a fundamental role analogous to secret keys in achieving secure communication. Hence, by defining two statistical measures of the channel sparsity, we analytically characterize its impact on secrecy rate. In particular, a substantial improvement on secrecy rate can be obtained by the proposed scheme due to the uncertainty, i.e., 'entropy', introduced by the channel sparsity which is unknown to the eavesdropper. It is revealed that sparsity in the power domain can always contribute to the secrecy rate. In contrast, in the angle domain, there exists an optimal level of sparsity that maximizes the secrecy rate. The effectiveness of the proposed scheme and derived results are verified by numerical simulations
Predictor Antenna Systems: Exploiting Channel State Information for Vehicle Communications
Vehicle communication is one of the most important use cases in the fifth
generation of wireless networks (5G). The growing demand for quality of service
(QoS) characterized by performance metrics, such as spectrum efficiency, peak
data rate, and outage probability, is mainly limited by inaccurate
prediction/estimation of channel state information (CSI) of the rapidly
changing environment around moving vehicles. One way to increase the prediction
horizon of CSI in order to improve the QoS is deploying predictor antennas
(PAs). A PA system consists of two sets of antennas typically mounted on the
roof of a vehicle, where the PAs positioned at the front of the vehicle are
used to predict the CSI observed by the receive antennas (RAs) that are aligned
behind the PAs. In realistic PA systems, however, the actual benefit is
affected by a variety of factors, including spatial mismatch, antenna
utilization, temporal correlation of scattering environment, and CSI estimation
error. This thesis investigates different resource allocation schemes for the
PA systems under practical constraints.Comment: Licentiate thesis, Chalmers University of Technolog
Predictor Antenna Systems: Exploiting Channel State Information for Vehicle Communications
Vehicle communication is one of the most important use cases in the fifth generation of wireless networks (5G).\ua0 The growing demand for quality of service (QoS) characterized by performance metrics, such as spectrum efficiency, peak data rate, and outage probability, is mainly limited by inaccurate prediction/estimation of channel state information (CSI) of the rapidly changing environment around moving vehicles. One way to increase the prediction horizon of CSI in order to improve the QoS is deploying predictor antennas (PAs).\ua0 A PA system consists of two sets of antennas typically mounted on the roof of a vehicle, where the PAs positioned at the front of the vehicle are used to predict the CSI observed by the receive antennas (RAs) that are aligned behind the PAs. In realistic PA systems, however, the actual benefit is affected by a variety of factors, including spatial mismatch, antenna utilization, temporal correlation of scattering environment, and CSI estimation error. This thesis investigates different resource allocation schemes for the PA systems under practical constraints, with main contributions summarized as follows.First, in Paper A, we study the PA system in the presence of the so-called spatial mismatch problem, i.e., when the channel observed by the PA is not exactly the same as the one experienced by the RA. We derive closed-form expressions for the throughput-optimized rate adaptation, and evaluate the system performance in various temporally-correlated conditions for the scattering environment. Our results indicate that PA-assisted adaptive rate adaptation leads to a considerable performance improvement, compared to the cases with no rate adaptation. Then, to simplify e.g., various integral calculations as well as different operations such as parameter optimization, in Paper B, we propose a semi-linear approximation of the Marcum Q-function, and apply the proposed approximation to the evaluation of the PA system. We also perform deep analysis of the effect of various parameters such as antenna separation as well as CSI estimation error. As we show, our proposed approximation scheme enables us to analyze PA systems with high accuracy.The second part of the thesis focuses on improving the spectral efficiency of the PA system by involving the PA into data transmission. In Paper C, we analyze the outage-limited performance of PA systems using hybrid automatic repeat request (HARQ). With our proposed approach, the PA is used not only for improving the CSI in the retransmissions to the RA, but also for data transmission in the initial round.\ua0 As we show in the analytical and the simulation results, the combination of PA and HARQ protocols makes it possible to improve the spectral efficiency and adapt transmission parameters to mitigate the effect of spatial mismatch
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