Papers by Roberto Lopez-valcarce
We present a reinitialization scheme for blind equalizers adapted via the constant modulus algori... more We present a reinitialization scheme for blind equalizers adapted via the constant modulus algorithm (CMA) when an all-pole prefilter is included to whiten the received signal. The mechanism exploits the special structure of the minimum mean squared error (MMSE) equalizers and their relation with CMA equalizers. A heuristic rule for blind determination of the best equalization delay is also provided.
On Blind Adaptive Algorithms For IIR Equalizers
Previous work has shown that the optimum MSE causalequalizer is IIR, with a number of poles equal... more Previous work has shown that the optimum MSE causalequalizer is IIR, with a number of poles equal to the channelorder. We investigate two different blind criteria forthe adaptation of the recursive part of the equalizer: OutputVariance Minimization (OVM) and a Pseudolinear Regression(PLR) method. In sufficient order cases both algorithmsconverge to the desired setting. In undermodeled cases (i.e.the number of poles
IEEE Transactions on Information Theory, 2015
The class of complex random vectors whose covariance matrix is linearly parameterized by a basis ... more The class of complex random vectors whose covariance matrix is linearly parameterized by a basis of Hermitian Toeplitz (HT) matrices is considered, and the maximum compression ratios that preserve all second-order information are derived -the statistics of the uncompressed vector must be recoverable from a set of linearly compressed observations. This kind of vectors typically arises when sampling widesense stationary random processes and features a number of applications in signal and array processing.
2014 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), 2014
Active interference cancellation (AIC) is an effective technique to shape the OFDM spectrum, prov... more Active interference cancellation (AIC) is an effective technique to shape the OFDM spectrum, providing deep notches over protected bands without affecting receiver design. However, AIC typically introduces spectrum overshoot, of concern if compliance with a spectral emission mask is required. Most AIC designs neglect these spectral spurs, focusing on minimization of out-of-band radiation. We present a novel design that explicitly takes spectral spurs into account. The resulting optimization problem is convex and can be efficiently solved, although for systems with a large number of subcarriers the computational effort can be significant. A suboptimal solution based on a previous AIC design with a maximum power constraint is also proposed. Its performance is close to optimal while obtained at a much lower computational cost, comparing favorably to previous schemes.
2012 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), 2012
Many problems in digital communications involve wideband radio signals. As the most recent exampl... more Many problems in digital communications involve wideband radio signals. As the most recent example, the impressive advances in Cognitive Radio systems make even more necessary the development of sampling schemes for wideband radio signals with spectral holes. This is equivalent to considering a sparse multiband signal in the framework of Compressive Sampling theory. Starting from previous results on multicoset sampling and recent advances in compressive sampling, we analyze the matrix involved in the corresponding reconstruction equation and define a new method for the design of universal multicoset codes, that is, codes guaranteeing perfect reconstruction of the sparse multiband signal.
Acoustics, Speech, and Signal Processing, 1988. ICASSP-88., 1988 International Conference on
Spectrum sensing is a challenging key component of the Cognitive Radio paradigm, since primary si... more Spectrum sensing is a challenging key component of the Cognitive Radio paradigm, since primary signals must be detected in the face of noise uncertainty and at signal-to-noise ratios (SNRs) well below decodability levels. Multiantenna detectors exploit spatial independence of receiver thermal noise to boost detection performance and robustness. Here, we study the problem of detecting Gaussian signals with unknown rank-P spatial covariance matrix when the noise at the receiver is independent across the antennas and with unknown power. A generic diagonal noise covariance matrix is allowed to model calibration uncertainties in the different antenna frontends. We derive the generalized likelihood ratio test (GLRT) for this detection problem. Although, in general, the corresponding statistic must be obtained by numerical means, in the low SNR regime the GLRT does admit a closed form. Numerical simulations show that the proposed asymptotic detector offers good performance even for moderat...
Spectrum sensing is a key component of the Cognitive Radio paradigm. Multiantenna detectors can e... more Spectrum sensing is a key component of the Cognitive Radio paradigm. Multiantenna detectors can exploit different spatial features of primary signals in order to boost detection performance and robustness in very low signal-to-noise ratios. However, in several cases these detectors require additional information, such as the rank of the spatial covariance matrix of the received signal. In this work we study the problem of estimating this rank under Gaussianity assumption using an uncalibrated receiver, i.e. with different (unknown) noise levels at each of the antennas.
2009 IEEE 10th Workshop on Signal Processing Advances in Wireless Communications, 2009
The cognitive radio paradigm is based on the ability to detect the presence of primary users in a... more The cognitive radio paradigm is based on the ability to detect the presence of primary users in a given frequency band. In this scenario a spectrum monitor may estimate the signal power levels of all frequency channels in the band of interest, together with the background noise level. We address Maximum Likelihood estimation for this problem, exploiting a priori knowledge about the primary network, summarized in the spectral shape of primary transmissions. An iterative asymptotic ML estimate is proposed, which can be further simplified in order obtain a computationally more efficient Least Squares estimator with performance very close to the Cramer-Rao lower bound in several cases of interest.
Oversampled filter bank for frequency estimation
2009 Proceedings of 6th International Symposium on Image and Signal Processing and Analysis, 2009
Abstract Subband frequency estimation has already been shown to yield better performance than ful... more Abstract Subband frequency estimation has already been shown to yield better performance than fullband frequency estimation. It should be noted, however that decimation stage in subband filtering imposes limitations on the performance of such filters. In fact, poor performance is obtained for frequencies close to the edges due to decimation. This paper proposes a new subband scheme based on oversampled filter bank for frequency estimation, which reduces the edge problem. Simulation results are included to valid the ...
Proceedings of the 36th IEEE Conference on Decision and Control, 1997
The standard equation-error system identification method is known to be biasecl if the data are n... more The standard equation-error system identification method is known to be biasecl if the data are noise corrupted. It is also known that in certain cases the poles of the estimate can lie outside the unit circle. In this paper conditions for the unknown plant guaranteeing a stable estimate are investigated.
Hyperstable polyphase adaptive IIR filters
1999 IEEE International Conference on Acoustics, Speech, and Signal Processing. Proceedings. ICASSP99 (Cat. No.99CH36258), 1999
... k=l With this, the adaptive algorithm can be written as with p > 0 a suitable stepsize. Le... more ... k=l With this, the adaptive algorithm can be written as with p > 0 a suitable stepsize. Lemma 1 kt ~ ( z ) = 1 + E,"=, CkZ-k. fer function C(z)/D,(zM) is SPR, ie C(z)/D,(z') is stable and causal and the trans-if the system Re c(.'w) > 0 \d w , D,(ejwM) ...
IEEE International Conference on Acoustics Speech and Signal Processing, 2002
A novel car speed estimation technique from a pair of omnidirectional microphones is presented. T... more A novel car speed estimation technique from a pair of omnidirectional microphones is presented. This approach is based on the maximum likelihood principle and it directly estimates the speed without any assumptions on the acoustic signal emitted by the vehicle. This has the advantages of bypassing troublesome intermediate delay estimation steps as well as eliminating the need for an accurate yet general enough acoustic model. An analysis of the estimate for the particular case of a narrowband source is provided. The estimation algorithm is well suited to DSP implementation and performs well with preliminary field data.
IEEE Communications Letters, 2014
Active interference cancellation (AIC) is a multicarrier spectrum sculpting technique which reduc... more Active interference cancellation (AIC) is a multicarrier spectrum sculpting technique which reduces the power of undesired out-of-band emissions by adequately modulating a subset of reserved cancellation subcarriers. In most schemes online complexity is a concern, and thus cancellation subcarriers have traditionally been constrained to linear combinations of the data subcarriers. Recent AIC designs truly minimizing out-ofband emission shift complexity to the offline design stage, motivating the consideration of more general mappings to improve performance. We show that there is no loss in optimality incurred by constraining these mappings to the set of linear functions.
2011 4th IEEE International Workshop on Computational Advances in Multi-Sensor Adaptive Processing (CAMSAP), 2011
The detection of bandlimited (BL) constant magnitude (CM) signals in white noise of unknown power... more The detection of bandlimited (BL) constant magnitude (CM) signals in white noise of unknown power is analyzed. This is relevant in the context of Dynamic Spectrum Access since Wireless Microphones (WM) typically use analog FM modulation when transmitting in the TV bands, and their bandwidth is much smaller than that of a TV channel. Although detectors exploiting either the BL or the CM properties have been presented in the literature, these two features have not been jointly considered yet. We derive the Generalized Likelihood Ratio test for this setting. Performance is evaluated in the framework for WM simulation developed to assist the IEEE 802.22 Working Group.
2011 IEEE 12th International Workshop on Signal Processing Advances in Wireless Communications, 2011
Detection of unknown signals with constant modulus (CM) using multiple antennas in additive white... more Detection of unknown signals with constant modulus (CM) using multiple antennas in additive white Gaussian noise of unknown variance is considered. The channels from the source to each antenna are assumed frequency-flat and unknown. This problem is of interest for spectrum sensing in cognitive radio systems in which primary signals are known to have the CM property. Examples include analog frequency modulated signals such as those transmitted by wireless microphones in the TV bands and Gaussian Minimum Shift Keying modulated signals as in the GSM cellular standard. The proposed detector, derived from a Generalized Likelihood Ratio (GLR) approach, exploits both the CM property and the spatial independence of noise, outperforming the GLR test for Gaussian signals as shown by simulation.
2013 IEEE International Conference on Acoustics, Speech and Signal Processing, 2013
With full-duplex relays, simultaneous reception and transmission in the same frequency produce se... more With full-duplex relays, simultaneous reception and transmission in the same frequency produce self-interference distortion and relay oscillation, hampering reception at the receiver end unless properly mitigated. Previous adaptive methods for self-interference cancellation using multiple receive antennas require knowledge of the direction of arrival of the source signal. We present a novel architecture and adaptive scheme for Amplify-and-Forward relays allowing cancellation without such knowledge. The temporal filter is updated under the property restoral paradigm in order to match the power spectrum of the retransmitted signal to that of the source signal.
2012 IEEE 13th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), 2012
In full-duplex relays, simultaneous reception and transmission in the same frequency results in s... more In full-duplex relays, simultaneous reception and transmission in the same frequency results in self-interference which distorts the retransmitted signal and makes the relay prone to oscillation. We present an adaptive feedback canceller for multiple-input single-output (MISO) relays, efficiently combining spatial and temporal processing. The receive array is adaptively steered towards the minimum variance distortionless response (MVDR), whereas the temporal filter update is based on a novel low-complexity spectrum shaping scheme which avoids introducing additional delay in the relay station.
The Proportional Fair Scheduler (PFS) has become a popular channel-aware resource allocation meth... more The Proportional Fair Scheduler (PFS) has become a popular channel-aware resource allocation method in wireless networks, as it effectively exploits multiuser diversity while providing fairness to users. PFS decisions on which mobile station (MS) to schedule next are based on Channel Quality Indicator (CQI) values. Since CQI values are reported by the MSs to the scheduler, network performance can be severely degraded if some malicious MSs report forged information. Previous approaches to this security issue are based either on modifying PFS, which may be undesirable in some contexts, or authenticating CQI reports by periodic transmission of challenges, which increases overhead. Instead, we propose to detect misreporting attackers, based on the time correlation features of the wireless channel. Our approach does not require scheduler modification, and it does not increase overhead. Simulation results under realistic settings are provided to show the effectiveness of the proposed test.
DISTRIBUTED AOA-BASED SOURCE POSITIONING IN NLOS WITH SENSOR NETWORKS
Active Interference Cancellation (AIC) techniques for OFDM spectrum sculpting have gained interes... more Active Interference Cancellation (AIC) techniques for OFDM spectrum sculpting have gained interest over the last years, and several extensions to the MIMO case have been recently proposed. However, these designs do not fully exploit the spatial diversity provided by the multiple transmit antennas, as canceler allocation is fixed. This paper proposes a more general mechanism for the allocation of the cancellation subcarriers across antennas in order to better exploit spatial diversity. In particular, we present a novel AIC design for cognitive MIMO-OFDM systems, in which transmit antennas compete against each other for a fixed number of cancellation subcarriers. We show that this more general allocation approach results in significant performance improvements with respect to previous designs.
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Papers by Roberto Lopez-valcarce