Performance Analysis of Channel Extrapolation in FDD Massive MIMO Systems

Channel estimation for the downlink of frequency division duplex (FDD) massive MIMO systems is well known to generate a large overhead as the amount of training generally scales with the number of transmit antennas in a MIMO system. In this paper, we consider the solution of extrapolating the channel frequency response from uplink pilot estimates to the downlink frequency band, which completely removes the training overhead. We first show that conventional estimators fail to achieve reasonable accuracy. We propose instead to use high-resolution channel estimation. We derive theoretical lower bounds (LB) for the mean squared error (MSE) of the extrapolated channel. Assuming that the paths are well separated, the LB is simplified in an expression that gives considerable physical insight. It is then shown that the MSE is inversely proportional to the number of receive antennas while the extrapolation performance penalty scales with the square of the ratio of the frequency offset and the training bandwidth. The channel extrapolation performance is validated through numeric simulations and experimental measurements taken in an anechoic chamber. Our main conclusion is that channel extrapolation is a viable solution for FDD massive MIMO systems if accurate system calibration is performed and favorable propagation conditions are present.Comment: arXiv admin note: substantial text overlap with arXiv:1902.0684

Interactions between Type 1 Interferons and the Th17 Response in Tuberculosis: Lessons Learned from Autoimmune Diseases

textabstractThe classical paradigm of tuberculosis (TB) immunity, with a central protective role for Th1 responses and IFN-γ-stimulated cellular responses, has been challenged by unsatisfactory results of vaccine strategies aimed at enhancing Th1 immunity. Moreover, preclinical TB models have shown that increasing IFN-γ responses in the lungs is more damaging to the host than to the pathogen. Type 1 interferon signaling and altered Th17 responses have also been associated with active TB, but their functional roles in TB pathogenesis remain to be established. These two host responses have been studied in more detail in autoimmune diseases (AID) and show functional interactions that are of potential interest in TB immunity. In this review, we first identify the role of type 1 interferons and Th17 immunity in TB, followed by an overview of interactions between these responses observed in systemic AID. We discuss (i) the effects of GM-CSF-secreting Th17.1 cells and type 1 interferons on CCR2+ monocytes; (ii) convergence of IL-17 and type 1 interferon signaling on stimulating B-cell activating factor production and the central role of neutrophils in this process; and (iii) synergy between IL-17 and type 1 interferons in the generation and function of tertiary lymphoid structures and the associated follicular helper T-cell responses. Evaluation of these autoimmune-related pathways in TB pathogenesis provides a new perspective on recent developments in TB research

Physical Layer Security in an OFDM Time Reversal SISO Communication With Imperfect Channel State Information

A frequency domain time-reversal (TR) precoder is proposed to perform physical layer security in single-input single-output (SISO) systems using orthogonal frequency-division multiplexing (OFDM) and artificial noise (AN) injection. This scheme guarantees the secrecy of a communication towards a legitimate user, Bob, by exploiting the frequency diversity selective behaviour in multipath channels. The transmitter, Alice, has imperfect channel state information (CSI) of the legitimate link thanks to the channel reciprocity in time division duplex systems and does not know the instantaneous CSI of a potential eavesdropper, Eve. Three optimal decoding structures at Eve are considered in a block fading environment depending on the handshake procedure between Alice and Bob. Closed-form approximations of the signal-to-noise ratio required at Bob and the maximal CSI error that can be made at Alice, in order to guarantee a communication ergodic secrecy rate (ESR), are derived. Furthermore, the optimal amount of AN energy to inject, considering imperfect CSI, is also given as a closed-form expression. A trade-off on the choice of the spreading factor of the TR precoder is established between maximizing the ESR and decreasing the ϵ−achievable secrecy rate. Finally, thanks to these results, Alice can be a priori aware of the ESR over which she can establish a secure communication

Efficient Equalization of Time-Varying Channels in MIMO OFDM Systems

The orthogonality of the cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) modulation is ensured as long as the channel can be assumed constant across the duration of one CP-OFDM symbol period. Unfortunately, this assumption may not hold anymore for a large variety of emerging scenarios with mobility, high carrier frequency and multiple carrier frequency offsets. To tackle this issue, we propose a novel equalization structure. In contrast to existing works in the literature, the equalizer is obtained by considering a Taylor approximation of the ideal time-varying channel equalizer function. This results in an extremely simple implementation only consisting of per-subcarrier multiplications and FFT/IFFT operations. The general form of the equalizer is particularized to two specific cases: zero forcing and linear minimum mean squared error. Furthermore, the implementation complexity of the equalizers is computed and an analytical formula is proposed to efficiently evaluate their performance. Finally, numerical results demonstrate the efficiency of the proposed receivers as compared to the ideal one and previous works

Iterative ToA-Based Localization of Wireless Transmitters Using Dirichlet-Kernel-Based Range Representation

Iterative localization is currently seen as an attractive solution to localize a transmitter in a cellular network. It has been shown that, by iterating between a range estimation step and a multi-lateration step, it is possible to refine the estimation in the first step, where only local information is used at iteration one. The iterative approach gets close to the performance of direct localization; nevertheless, it does not seem to converge to the direct localization performance for medium and low signal-to-noise-ratio values, due to the fact that it still suffers from loss of information due to projections and data representation. In this work, we propose to approximate the range log-likelihood at the base station with a Dirichlet kernel and to perform all the processing in a common xy-domain so that projections are no longer needed. We numerically show that our approach brings significant performance gains as compared to the time-of-arrival based iterative position estimation algorithm, getting really close to the performance of direct localization

Secrecy capacity of FBMC-OQAM modulation over frequency selective channel

This letter studies the Information-theoretic secrecy capacity of an Offset-QAM-based filterbank multicarrier (FBMC-OQAM) communication over a wiretap frequency selective channel. The secrecy capacity Is formulated as an optimization problem which has a closed-form solution In the high signal-to-noise ratio (SNR) regime. Two of the most common equalization strategies In FBMC-OQAM are considered, namely, single-tap and multi-tap equalization. For the sake of comparison, we also consider the secrecy capacity of a generic modulation and a cyclic prefix-orthogonal frequency division multiplexing (CP-OFDM) modulation. As a result, we find that FBMC-OQAM Is particularly competitive for medium-to-long burst transmissions.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Semi-empirical Model of Global Exposure using Stochastic Geometry

Exposure to electromagnetic fields due to cellular networks in an urban environment is studied using stochastic geometry. We describe a simple but functional and realistic semi-empirical model based on the modeling of base station patterns as Poisson Point Processes and applied to two Brussels municipalities. Using this model, a study of the impact of base station densification is performed according to two theoretical scenarios. From simple assumptions, it is concluded that global exposure is expected to remain approximately constant in the case of intelligent network evolution involving the densification of macro cells and the addition of a network of low-power small cells