David Gregoratti

CTTC Catalan Telecommunications Technology Centre, Barcino, Catalonia, Spain

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Publications (25)20.66 Total impact

  • Xavier Mestre · David Gregoratti

    No preview · Article · Jan 2015 · IEEE Transactions on Signal Processing
  • David Gregoratti · Xavier Mestre
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    ABSTRACT: An AF relay splits an FBMC transmission link into a two-hop channel. The receiver implements a parallel multistage FBMC demodulator/equalizer that is suitable for fading channels whose frequency response cannot be considered flat over the subcarriers. The signal quality at the output of such a receiver is characterized in terms of signal-to-noise-plus-distortion ratio (SNDR), that takes into account the residual interference after equalization and the effect of the noise collected on both hops. The SNDR is then used to estimate the spectral efficiency of the link. The comparison with the direct link shows that the AF-relay scheme offers some gain, especially in lossy environments with high path-loss exponent.
    No preview · Conference Paper · Jun 2014
  • Xavier Mestre · David Gregoratti
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    ABSTRACT: The problem of MIMO transmission using filterbank multicarrier (FBMC) modulations in strong frequency selective channels is considered. A novel architecture for the implementation of MIMO precoders and linear receivers is derived, which consists of multiple parallel stages that are combined at the per-subcarrier level. Each of these stages is constructed like a classical FBMC modulator/demodulator, using the successive derivatives of the prototype pulse instead of the original one. The performance of the proposed architecture is theoretically characterized in terms of the residual distortion power at the output of the receiver, assuming an asymptotically large number of subcarriers. Results demonstrate the effectiveness of the proposed architecture in MIMO channels with severe frequency selectivity.
    No preview · Conference Paper · May 2014
  • J. Matamoros · C. Antón-Haro · D. Gregoratti · L. Berbakov
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    ABSTRACT: In this paper, we propose two iterative synchronization schemes for distributed beamforming (DBF) with sidelobe control. The scenario that we address is that of a wireless sensor network surrounded by one intended base station (BS) and multiple unintended ones (serving other WSNs). We consider two specific synchronization schemes with soft and hard constraints in terms of the received signal strength acceptable at the unintended base stations. The schemes merely require partial CSI at the BSs plus the broadcast of one-bit feedback messages from the intended BS messages to the sensor nodes.
    No preview · Article · Jan 2014
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    David Gregoratti · Javier Matamoros
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    ABSTRACT: In this paper, a distributed convex optimization framework is developed for energy trading between islanded microgrids. More specifically, the problem consists of several islanded microgrids that exchange energy flows by means of an arbitrary topology. Due to scalability issues and, in order to safeguard local information on cost functions, a subgradient-based cost minimization algorithm is proposed that converges to the optimal solution in a practical number of iterations and with a limited communication overhead. Furthermore, this approach allows for a very intuitive economics interpretation that explains the algorithm iterations in terms of "supply--demand model" and "market clearing". Numerical results are given in terms of convergence rate of the algorithm and attained costs for different network topologies.
    Preview · Article · Jul 2013 · IEEE Transactions on Industrial Electronics
  • Xavier Mestre · David Gregoratti
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    ABSTRACT: A point-to-point multiple-relay communication system with half-duplex constraints is considered. The relays operate under the amplify-and-forward paradigm and implement a linear dispersion distributed space-time code with randomized dispersion matrices of independent and identically distributed entries. The large-signal-to-noise-ratio behavior of the asymptotically large deterministic system is studied for both the optimum (maximum likelihood) receiver and the linear minimum mean squared error (LMMSE) receiver. The diversity order of the system is shown to have a strong dependence on the aspect ratio of the linear dispersion matrices. More specifically, when the linear dispersion matrices are sufficiently tall, both receivers achieve the maximum diversity order, i.e., the total number of relays plus one. Conversely, when the linear dispersion matrices are fat (in the sense that relays linearly compress the information received from the source), the optimum receiver is shown to achieve an asymptotic diversity order of two, whereas the LMMSE receiver is totally unable to exploit the available spatial diversity.
    No preview · Article · May 2013 · IEEE Transactions on Information Theory
  • D. Gregoratti · J. Matamoros
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    ABSTRACT: In this paper, a distributed convex optimization framework is developed to manage energy flows between is-landed microgrids. More specifically, the problem consists of two islanded microgrids with the additional capability of selling energy to one another. In order to avoid a central controller and to reduce communications requirements, a subgradient-based cost minimization algorithm is proposed that converges to the centralized solution in a practical number of iterations. Furthermore, this approach allows for a very intuitive, economics interpretation that explains the algorithm iterations in terms of “supply-demand model” and “market clearing”. Finally, numerical results show that microgrid cooperation brings a benefit, both globally (system level) and locally (microgrid level).
    No preview · Conference Paper · Jan 2013
  • Javier Matamoros · David Gregoratti · Mischa Dohler
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    ABSTRACT: In this paper, we address the case where two microgrids are isolated from the main power grid but can exchange energy with each other in a peer-to-peer (P2P) manner. The goal is to minimize the total cost resulting from energy generation and transportation, while each microgrid satisfies its local power demand. We first propose a centralized solution. In this approach, a central controller must have access to all the information. For those situations where privacy must be protected, we propose a distributed approach, in which each microgrid solves a local optimization problem in an iterative fashion by exchanging some information with the other one.
    No preview · Conference Paper · Nov 2012

  • No preview · Chapter · May 2012
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    David Gregoratti · Walid Hachem · Xavier Mestre
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    ABSTRACT: This paper presents a randomized linear-dispersion space-time block code for decode-and-forward synchronous relays. The coding matrices are obtained as a set of columns (or rows) of randomly generated Haar-distributed unitary matrices. With respect to independent and identically distributed (i.i.d.)-generated codes, this particular isometric structure reduces the intersymbol interference generated within each relay. The gain over i.i.d. codes in terms of spectral efficiency is analyzed for both the LMMSE and the ML receivers under the assumption of frequency-flat quasi-static fading. In this setting, the spectral efficiency is a random quantity, since it depends on the random coding matrices. However, it is proven that the spectral efficiency converges in probability to a deterministic quantity when the dimensions of the matrices tend to infinity while keeping constant their ratio, i.e., the coding rate α. Consequently, when the random coding matrices are large enough, the presented system behaves as a deterministic one. This result is achieved by means of the rectangular R-transform, a powerful tool of free probability theory which allows determining the distribution of the singular values of a sum of rectangular matrices.
    Preview · Article · Feb 2012 · IEEE Transactions on Signal Processing
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    David Gregoratti · Xavier Mestre
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    ABSTRACT: A simple half-duplex decode-and-forward relay channel is presented and analyzed. Relays access the common channel by means of a randomized linear-dispersion space-time block code which is flexible with respect to the number of relays and the coding rate α. When the dimensions of the linear dispersion matrices grow large, but with constant ratio α, the spectral efficiency of the system converges fast to a deterministic quantity. Simulation results show that this asymptotic value is an extremely good approximation of the finite reality, even for not-so-large codes. Then, this asymptotic spectral efficiency is used to characterize the outage probability in the high-SNR regime. With maximum-likelihood reception, the proposed randomized coding scheme is shown to achieve full diversity order L+1, with L the total number of relays. On the contrary, with the sub-optimal LMMSE receiver, relays add diversity to the system only if the coding rate is small enough.
    Preview · Article · Aug 2011 · IEEE Transactions on Wireless Communications
  • David Gregoratti · Xavier Mestre

    No preview · Article · Jan 2011
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    David Gregoratti · Walid Hachem · Xavier Mestre
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    ABSTRACT: A distributed random linear-dispersion space-time block coding for relay networks is presented. The coding matrix assigned to each relay is built by a set of columns (or rows) of an independent random Haar-distributed unitary matrix. Within each transmitter, the particular structure of the codes allows to reduce interference in the multiplexed symbols, with respect to similar codes with random i.i.d. entries. The system is shown to have an asymptotically deterministic behavior (the convergence is in probability) as the dimensions of the code grow large while keeping constant the coding rate. The limiting performance can be characterized by means of free-probability theory: more specifically, the rectangular R-transform is employed to compute the asymptotic distribution of the singular values of a sum of rectangular matrices, which completely characterizes the asymptotic spectral efficiency of the corresponding channel.
    Preview · Conference Paper · Jun 2010
  • J. Alonso-Zarate · J. Matamoros · D. Gregoratti · M. Dohler
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    ABSTRACT: Introduction This chapter reviews the emerging paradigm of machine-to-machine (M2M) communications in the context of smart grids. Commencing here with an introduction to the topic at hand, we then introduce in subsequent sections available M2M communications technologies as well as the applicability of said technologies. We then dwell in greater detail on M2M architectural standards bodies, such as ETSI M2M and 3GPP MTC. We finally position the use of M2M in smart grids and identify open challenges for a symbiotic development of both technologies. A machine-to-machine network is defined to be a network formed by devices that communicate with each other without (or with very little) human intervention in order to accomplish some specific task(s). The prime driver for this networking paradigm is the ability of a large number of devices/machines to execute tasks in an autonomous (and often distributed) manner which is beyond the ability of humans. From a technical point of view, it requires the system to be scalable, power-efficient, autonomous, intelligent; among many other properties, some of which are discussed below. Indeed, as highlighted throughout this book as well as below in Section 6.5, a huge number of points in the power grid need to be constantly monitored and controlled to ensure smart operation of the system. Although the above design aims have been conceptually the core to various prior networking design efforts, the idea of M2M is currently receiving great attention from both academia and industry.
    No preview · Article · Jan 2010
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    ABSTRACT: We present in this paper the analytical evaluation of a simple mechanism to reduce the average transmission delay of master stations in a network based on the Distributed Queuing MAC protocol for Ad hoc Networks (DQMAN). When DQMAN is executed, the network is self-organized into dynamic and spontaneous master-slave clusters. Within each cluster, a high performance MAC protocol based on a tree-splitting collision resolution algorithm which uses access minislots is executed. By allowing temporary master stations to avoid contention to get access to the channel, their average packet transmission delay can be effectively reduced compared to that of slaves. This technique provides thus master stations with higher access priority to the channel and, indeed, could be used in any MAC protocol based on access minislots to provide a subset of users with higher priority.
    Full-text · Conference Paper · Sep 2009
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    David Gregoratti · Xavier Mestre
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    ABSTRACT: This paper analyzes a cooperative communications system where a set of decode and forward relays provides support to a point to point communication. Each relay linearly transforms the K received symbols, if correctly decoded, into N new ones to be sent to the destination. The coding matrices are randomly generated, making the system simple to design, scalable and robust to synchronism errors. It is shown that such a random code, together with the optimal maximum-likelihood receiver, achieves full diversity for any value of the ratio alpha = K/N. This is not true for the linear minimum mean square error receiver, which loses diversity when alpha increases beyond a given threshold. To avoid dealing with the randomness of the code, the analysis is carried out in the asymptotic domain, i.e. for K and N growing without bound but with ratio converging to a finite quantity alpha. Random matrix theory results are used to derive this asymptotic approximation.
    Preview · Conference Paper · Aug 2009
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    ABSTRACT: The analysis of an access priority mechanism for a high-performance medium access control (MAC) protocol, the distributed queueing MAC protocol for wireless ad hoc networks (DQMAN), is presented in this letter. DQMAN is comprised of a hierarchical, dynamic, and spontaneous master-slave clustering algorithm together with an embedded tree-splitting collision resolution algorithm based on access mini-slots. The responsibility of being master entails extra functionality, and thus extra energy consumption. Therefore, this responsibility must be shared in a dynamic manner among all the stations of the network in order to ensure fairness in the system. By allowing those stations acting as master stations to avoid contention to get access to the channel, their average packet transmission delay can be effectively reduced compared to that of slave stations. Consequently, stations may be encouraged to operate in master mode regardless of the extra functions they may have to carry out. We analyze in this letter the reduced average packet transmission delay for masters.
    Full-text · Article · Aug 2009 · IEEE Communications Letters
  • D. Gregoratti · X. Mestre
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    ABSTRACT: This paper considers a cooperative communications system where a single relay aids a point-to-point transmission using an amplify and forward configuration. The relay linearly encodes the K received symbols into N new ones by means of a matrix multiplication, and the destination employs a LMMSE filter to estimate the information originally transmitted. A large- SNR approximation of the outage probability P<sub>out</sub> is derived for this system. The analysis of P<sub>out</sub> shows that the diversity order of the system depends on the ratio K/N: if such a ratio is too high (the relay excessively compresses the information), no diversity is added by introducing the relay. Conversely, if this ratio is smaller than a certain quantity, diversity order 2 is ensured. The derived expressions are finally used to establish the diversity-multiplexing tradeoff of the system.
    No preview · Conference Paper · Jul 2009
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    David Gregoratti · Xavier Mestre
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    ABSTRACT: This paper presents a communication system where point-to-point transmissions are aided by L half-duplex, amplify and forward relays which access a common channel by means of random spreading matrices. During the relaying phase, the source can either remain silent or send new symbols. The two cases are compared in terms of spectral efficiency, showing that the latter is always preferable. Next, a sufficient condition for the superiority of the presented relaying scheme over the simple direct-link channel is derived. Particularizing to the one relay case, this condition turns out to be also necessary and allows to identify which is the best time-sharing strategy between relay receiving and transmitting phases. We conclude the paper by discussing the suboptimal linear-minimum-mean-square-error receiver, which is known to be the linear filter that guarantees the maximum signal to noise and interference ratio at its output. To avoid dealing with the randomness introduced by the spreading matrices, the analysis is carried out in the asymptotic regime, i.e. when letting the number of transmitted symbols and the signature length grow without bound but with constant ratio. Under these hypotheses, indeed, random matrix theory results show that it is possible to derive deterministic almost sure equivalents which are excellent approximations of the finite reality.
    Preview · Article · Mar 2009 · IEEE Transactions on Wireless Communications
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    David Gregoratti · Xavier Mestre
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    ABSTRACT: Cooperative transmission is a convenient means of generating spa-tial diversity without the need of collocating multiple antennas in a single terminal. In this paper, the asymptotic diversity properties of a cooperative transmission system are investigated. The analy-sis is carried out for an amplify-and-forward multi-relay protocol implementing a distributed randomized space-time block code. Ac-cording to this approach, each relay multiplies the received signal by its specific spreading matrix, which is selected as having random independent and identically distributed entries. It is shown that the asymptotic diversity order of this system can vary from 2 to the total number of relays plus one. Furthermore, the achieved diversity or-der critically depends on the quotient between the number of rows and columns of the spreading matrices, which need to be sufficiently tall in order to guarantee full diversity of the system.
    Preview · Article · Jan 2009