David Gregoratti

CTTC Catalan Telecommunications Technology Centre, Barcino, Catalonia, Spain

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Publications (18)12.84 Total impact

  • 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.
    2014 European Conference on Networks and Communications (EuCNC); 06/2014
  • 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.
    ICASSP 2014 - 2014 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP); 05/2014
  • 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.
    IEEE Transactions on Information Theory 05/2013; 59(5):2936-2959. DOI:10.1109/TIT.2013.2242115 · 2.65 Impact Factor
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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.
    IEEE Transactions on Signal Processing 02/2012; DOI:10.1109/TSP.2011.2171685 · 3.20 Impact Factor
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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.
    IEEE Transactions on Wireless Communications 08/2011; DOI:10.1109/TWC.2011.050511.100338 · 2.76 Impact Factor
  • David Gregoratti, Xavier Mestre
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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.
    Wireless Pervasive Computing (ISWPC), 2010 5th IEEE International Symposium on; 06/2010
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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.
    Information Theory, 2009. ISIT 2009. IEEE International Symposium on; 08/2009
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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.
    IEEE Communications Letters 08/2009; 13(7-13):495 - 497. DOI:10.1109/LCOMM.2009.081502 · 1.46 Impact Factor
  • 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.
    Communications, 2009. ICC '09. IEEE International Conference on; 07/2009
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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.
    IEEE Transactions on Wireless Communications 03/2009; DOI:10.1109/TWC.2009.080314 · 2.76 Impact Factor
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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.
    Proceedings of the IEEE 20th International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC 2009, 13-16 September 2009, Tokyo, Japan; 01/2009
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    David Gregoratti, Xavier Mestre
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    ABSTRACT: This paper considers a DS/CDMA relay system where random spreading sequences are employed at the relays to allow multiple access to the channel between relays and destination. Two different protocols are considered and compared, the difference between them being whether the source remains silent or sends new information symbols during the relaying. For both protocols, we provide a low source power approximation for the spectral efficiency when employing, at the destination, either the optimal or the linear minimum mean square error receiver. The new expression is studied for the single relay case to identify under which conditions relaying results convenient over direct source-destination communication. Furthermore, we investigate the optimum percentage of relaying time in order to maximize the global spectral efficiency.
    Signal Processing Advances in Wireless Communications, 2008. SPAWC 2008. IEEE 9th Workshop on; 08/2008
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    D. Gregoratti, X. Mestre
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    ABSTRACT: This paper considers a simple CDMA-based amplify and forward half-duplex relay transmission scheme. The spectral efficiency of the system is computed for both the optimal and the linear minimum mean square error receivers. These expressions are then approximated with their asymptotic equivalents, assuming that the number of source symbols K and signature length N are infinitely large but with constant ratio alpha = K/N. Asymptotics are then compared with the spectral efficiency of the direct link to identify conditions that guarantee the superiority of relaying. We show that, for one relay and optimal receiver, the sufficient condition is also necessary. Finally, in the low-power regime, we compute the value of alpha that maximizes the information transfer: this coincides with the best time-sharing strategy between source and relay transmission.
    Wireless Pervasive Computing, 2008. ISWPC 2008. 3rd International Symposium on; 06/2008
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    D. Gregoratti, X. Mestre
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    ABSTRACT: This paper considers an amplify and forward DS/CDMA cooperative relay system with half-duplex relays. Given different receive structures for this scenario, the asymptotic spectral efficiency is analyzed assuming that the spreading factor and the information block length go to infinity at the same rate. The objective of this analysis is to identify the cases where transmission diversity, introduced by the relays, compensates for the physical-rate loss caused by the fact that the source terminal remains silent during the relay transmission. Results show that, provided that the source-destination channel amplitude is sufficiently low, there exists an optimum assignment of transmission time between source and relays.
    Signals, Systems and Computers, 2007. ACSSC 2007. Conference Record of the Forty-First Asilomar Conference on; 12/2007
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    David Gregoratti, Xavier Mestre
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    ABSTRACT: This paper considers a relay communication system where cooperative diversity is achieved by spreading the relay transmissions using direct-sequence CDMA (DS/CDMA). We derive expressions for the signal to noise ratio (SNR) employing different receiver filters. The relay signatures are generated according to a random model and the SNR is evaluated in the asymptotic domain, assuming that the number of relays and the signature length grow without bound but with constant ratio. We will also see how the introduction of the signatures, together with the use of the linear minimum square error receiver, effectively transform the relay channel in a set of parallel connections, thus fully exploiting cooperative diversity.
    Communications, 2007. ICC '07. IEEE International Conference on; 07/2007
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    David Gregoratti, Xavier Mestre
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    ABSTRACT: This paper compares the effects of an amplify and forward relay and a decode and forward relay on a point to point transmission system. Specifically, the outage probability and the consequent diversity order are derived in both cases, under the assumption of Rayleigh fading. The relay is assumed be half duplex and the communication is divided into two phases: the source transmits K symbols in the first one and the relay re-transmits N symbols in phase 2, with the source remaining silent. The relay symbols are obtained by linear transformation by means of a N × K randomly generated matrix. Accurate deterministic approximations (independent of the matrix statistics) are derived in the asymptotic domain, i.e. for K and N growing without bound but with constant ratio.
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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.