Phase-noise limitations in continuous-variable quantum key distribution with homodyne detection

In continuous-variables quantum key distribution with coherent states, the advantage of performing the detection by using standard telecoms components is counterbalanced by the lack of a stable phase reference in homodyne detection due to the complexity of optical phase-locking circuits and to the unavoidable phase noise of lasers, which introduces a degradation on the achievable secure key rate. Pilot-assisted phase-noise estimation and postdetection compensation techniques are used to implement a protocol with coherent states where a local laser is employed and it is not locked to the received signal, but a postdetection phase correction is applied. Here the reduction of the secure key rate determined by the laser phase noise, for both individual and collective attacks, is analytically evaluated and a scheme of pilot-assisted phase estimation proposed, outlining the tradeoff in the system design between phase noise and spectral efficiency. The optimal modulation variance as a function of the phase-noise amount is derived

Gaussian states and geometrically uniform symmetry

Quantum Gaussian states can be considered as the majority of the practical quantum states used in quantum communications and more generally in quantum information. Here we consider their properties in relation with the geometrically uniform symmetry, a property of quantum states that greatly simplifies the derivation of the optimal decision by means of the square root measurements. In a general framework of the $N$-mode Gaussian states we show the general properties of this symmetry and the application of the optimal quantum measurements. An application example is presented, to quantum communication systems employing pulse position modulation. We prove that the geometrically uniform symmetry can be applied to the general class of multimode Gaussian states

SINR degradation in MIMO-OFDM systems with channel estimation errors and partial phase noise compensation

The phase noise effect in multiple-input-multipleoutput systems employing orthogonal frequency division multiplexing is considered in a realistic scenario where the estimated channel matrix is affected by an error. The analytical SINR degradation due to phase noise and channel estimation is obtained for linear receivers (ZF and MMSE)This work is partly funded by the projects "COMONSENS" CSD2008- 00010 and "MULTI-ADAPTIVE" TEC2008-06327-C03-02Publicad

Effect of multipath and antenna diversity in MIMO-OFDM systems with imperfect channel estimation and phase noise compensation

The effect of phase noise in multiple-input–multiple-output systems employing orthogonal frequency division multiplexing is analyzed in a realistic scenario where channel estimation is not perfect, and the phase noise effects are only partially compensated. In particular, the degradation in terms of SNR is derived and the effects of the receiver and channel parameters are considered, showing that the penalty is different for different receiver schemes. Moreover it depends on the channel characteristics and on the channel estimation error. An analytical expression is used to evaluate the residual inter-channel interference variance and therefore the degradation. The effects of multipath and antenna diversity are shown to be different for the two types of linear receivers considered, the zero-forcing scheme and the minimum mean squared error receiver.This work has been partly funded by projects “MACAWI” TEC2005-07477-C02-02 and “MULTI-ADAPTIVE” TEC2008-06327-C03-02.Publicad

Joint channel and phase noise compensation for OFDM in fast fading multipath applications

Orthogonal frequency-division multiplexing (OFDM) systems, such as the new wireless standards or the schemes proposed for third-generation (3G) evolution, exhibit great sensitivity to the effects of phase noise and the time-varying propagation channel, which can introduce interchannel interference (ICI) due to the loss of orthogonality among subcarriers. In this paper, joint channel estimation and ICI reduction schemes are investigated, which compensate the effects of phase noise and multipath channel in a realistic scenario, where the channel is not assumed perfectly known, whereas its estimation is obtained by combining a decision-feedback scheme and a pilot-aided estimator. We propose a technique for ICI compensation that has reduced complexity by only considering the most significant terms of ICIPublicad

Analysis of SVD-Based Hybrid Schemes for Massive MIMO with Phase Noise and Imperfect Channel Estimation

In hybrid analog-digital schemes, proposed to reduce the number of RF chains especially at millimeter waves, the precoding at the transmitter and the combining at the receiver are split into digital and analog parts. We analyze the sensitivity of hybrid schemes to phase noise and channel estimation errors and we compare them to a full-digital approach. The scheme adopted for the analog part employs fixed phase shifters, then the digital part is optimized by a singular-value decomposition. We derive analytical expressions for the interference and the SNR degradation arising from the imperfect decomposition due to phase noise and the channel estimation error, for typical millimter-wave massive MIMO channels. In particular we show that when the channel estimation is made in the beam-space, this hybrid scheme is more robust to the phase noise and to the channel estimation errors than a full-digital approach

Constrained power allocation schemes for coordinated base station transmission using block diagonalization

In this study, we propose several power allocation schemes in a coordinated base station downlink transmission with per antenna and per base station power constraints. Block Diagonalization is employed to remove interference among users. For each set of power constraints, two schemes based on the waterfilling distribution are proposed and compared to the optimal solution, which can only be obtained numerically by using convex optimization. We show that the proposed schemes achieve a performance, in terms of weighted sum rate, very close to the optimal, without the heavy computational complexity required by the latter. The sum rates are compared first in a simplified two-user two-cell case where we also compare our approach to the previous solutions available in the literature. Then, we examine the performance in a multi-cell scenario where we also evaluate the degradation of the performance caused by imperfect channel state informationThis study has been partly funded by projects TEC2008-06327-C03-02, CCG10-UC3M/TIC-4620, and CSD2008-00010.Publicad

Mean Achievable Rates in Clustered Coordinated Base Station Transmission with Block Diagonalization

We focus on the mean achievable rate per user of the coordinated base station downlink transmission in a clustered cellular environment, with transmit power constraints at the base stations. Block Diagonalization is employed within the cluster to remove interference among users while the interference from other clusters remains. The average achievable rate per user is evaluated considering the effects of the propagation channel and the interference and a theoretical framework is presented to provide its analytical expression, validated by simulation results with different power allocation schemes. As an application, the number of cells of the cluster that maximizes the mean achievable rate per user is investigated. It can be seen that in most of the cases a reduced cluster size, close to seven cells, guarantees a rate very close to the maximum achievableThis work is partly funded by projects "GRE3N": TEC2011-29006-C03-03 and "COMONSENS": CSD2008-00010En-prens

Partial coordination in clustered base station MIMO transmission

This proceeding at: IEEE Wireless Communications and Networking Conference (WCNC, 2013), took place 2013, April, 7-10, in Shaghai (China)We present partial coordination strategies in a clustered cellular environment, evaluating the achievable rate in the downlink transmission. Block Diagonalization is employed for the coordinated users within the cluster to remove interference, while the interference from non-coordinated users remains. The achievable rate is evaluated resorting to an analytical expression conditioned on the position of the users in the cluster. A partial coordination approach is proposed to reduce the coordination complexity and overhead, where users close to the base station are not coordinated. Two approaches are considered, namely the non-coordinated users can be grouped and assigned separated resources from the coordinated ones, or they can be mixed.This work was supported by projects CSD2008-00010 “COMONSENS” and TEC2011-29006-C03-03 “GRE3N”

Achievable rate and fairness in coordinated base station transmission

This work focuses on the fairness in the distribution of the achievable rate per user in a cellular environment where clusters of base stations coordinate their transmissions in the downlink. Block Diagonalization is employed within the cluster to remove interference among users while the interference coming from other clusters remains. The probability distribution of the achievable rate per user shows a perfect match with a Gamma distribution so that a characterization in terms of mean and variance can provide a useful tool for the design of the clusters and the implementation of fairness strategies in a coordinated base station network with Block Diagonalization.This work is partly funded by the projects “GRE3N”: TEC2011-29006- C03-03, and “COMONSENS”: CSD2008-00010.Publicad