@phdthesis {437,
	title = {Cooperative Techniques for Interference Management in Wireless Networks},
	year = {2015},
	month = {May},
	school = {University of Cantabria},
	author = {Lameiro, Christian}
}
@article {484,
	title = {Spatial interference shaping for underlay MIMO cognitive networks},
	journal = {Signal Processing},
	volume = {134},
	year = {2017},
	pages = {174-184},
	doi = {j.sigpro.2016.12.012},
	author = {Lameiro, Christian and Utschick, Wolfgang and Santamar{\'\i}a, Ignacio}
}
@article {547,
	title = {Robust Improper Signaling for Two-user SISO Interference Channels},
	journal = {IEEE Transactions on Communications},
	volume = {67},
	year = {2019},
	month = {July},
	pages = {4709-4723},
	abstract = {It has been shown that improper Gaussian signaling (IGS) can improve the performance of wireless interference limited systems when perfect channel state information (CSI) is available. In this paper, we investigate the robustness of IGS against imperfect CSI on the transmitter side in a two-user single-input single-output (SISO) interference channel (IC) as well as in a SISO Z-IC, when interference is treated as noise. We assume that the true channel coefficients belong to a known region around the channel estimates, which we call the uncertainty region. Following a worst-case robustness approach, we study the rate-region boundary of the IC for the worst channel in the uncertainty region. For the two-user IC, we derive a robust design in closed-form, which is independent of the phase of the channels by allowing only one of the users to transmit IGS. For the Z-IC, we provide a closed-form design for the transmission parameters by considering an enlarged uncertainty region and allowing both users to employ IGS. In both cases, the IGS-based designs are ensured to perform no worse than proper Gaussian signaling. Furthermore, we show, through numerical examples, that the proposed robust designs significantly outperform non-robust solutions.},
	keywords = {Achievable rate region, imperfect CSI, improper Gaussian signaling, two-user interference channel, worst-case robustness.},
	doi = {10.1109/TCOMM.2019.2910549},
	author = {Mohammad Soleymani and Lameiro, Christian and Santamar{\'\i}a, Ignacio and Schreier, Peter J.}
}
@article {487,
	title = {Rate Region Boundary of the SISO Z-interference Channel with Improper Signaling},
	journal = {IEEE Transactions on Communications},
	volume = {65},
	year = {2017},
	month = {March},
	pages = {1022-1024},
	abstract = {This paper provides a complete characterization of the boundary of an achievable rate region, called the Pareto boundary, of the single-antenna Z interference channel (Z-IC), when interference is treated as noise and users transmit complex Gaussian signals that are allowed to be improper. By considering the augmented complex formulation, we derive a necessary and sufficient condition for improper signaling to be optimal. This condition is stated as a threshold on the interference channel coefficient, which is a function of the interfered user rate and which allows insightful interpretations into the behavior of the achievable rates in terms of the circularity coefficient (i.e., degree of impropriety). Furthermore, the optimal circularity coefficient is provided in closed form. The simplicity of the obtained characterization permits interesting insights into when and how improper signaling outperforms proper signaling in the singleantenna Z-IC. We also provide an in-depth discussion on the
optimal strategies and the properties of the Pareto boundary.},
	issn = {0090-6778},
	doi = {10.1109/TCOMM.2016.2641948},
	author = {Lameiro, Christian and Santamar{\'\i}a, Ignacio and Schreier, Peter J.}
}
@article {407,
	title = {A Quadratically Convergent Method for Interference Alignment in MIMO Interference Channels},
	journal = {IEEE Signal Processing Letters},
	volume = {21},
	year = {2014},
	month = {November},
	pages = {1423-1427},
	keywords = {alternating minimization, Gauss-Newton, interference alignment, interference channel, steepest descent},
	doi = {10.1109/LSP.2014.2338132},
	author = {Gonz{\'a}lez, {\'O}scar and Lameiro, Christian and Santamar{\'\i}a, Ignacio}
}
@article {565,
	title = {Maximally improper signaling in underlay MIMO cognitive radio networks},
	journal = {IEEE Transactions on Signal Processing},
	year = {2019},
	abstract = {In this paper we address the problem of subspace averaging, with special emphasis placed on the question of estimating the dimension of the average. The results suggest that the enumeration of sources in a multi-sensor array, which is a problem of estimating the dimension of the array manifold, and as a consequence the number of radiating sources, may be cast as a problem of averaging subspaces. This point of view stands in contrast to conventional approaches, which cast the problem as one of identifiying covariance models in a factor model. We present a robust formulation of the proposed order fitting rule based on majorization-minimization algorithms. A key element of the proposed method is to construct a bootstrap procedure, based on a newly proposed discrete distribution on the manifold of projection matrices, for stochastically generating subspaces from a function of experimentally-determined eigenvalues. In this way, the proposed subspace averaging (SA) technique determines the order based on the eigenvalues of an average projection matrix, rather than on the likelihood of a covariance model, penalized by functions of the model order. By means of simulation examples, we show that the proposed SA criterion is especially effective in high-dimensional scenarios with low sample support.},
	doi = {10.1109/TSP.2019.2953665},
	author = {Lameiro, Christian and Santamar{\'\i}a, Ignacio and Schreier, Peter J. and Utschick, Wolfgang}
}
@article {506,
	title = {Information-Theoretic Analysis of a Family of Improper Discrete Constellations},
	journal = {Entropy},
	volume = {20},
	year = {2018},
	doi = { doi:10.3390/e20010045},
	url = {http://www.mdpi.com/1099-4300/20/1/45/pdf},
	author = {Santamar{\'\i}a, Ignacio and Crespo, P. and Lameiro, Christian and Schreier, Peter J.}
}
@article {559,
	title = {Improper Signaling for SISO Two-user Interference Channels with Additive Asymmetric Hardware Distortion},
	journal = {IEEE Transactions on Communications},
	volume = {67},
	year = {2019},
	month = {December},
	pages = {8624-8638},
	abstract = {Hardware non-idealities are among the main performance restrictions for upcoming wireless communication systems. Asymmetric hardware distortions (HWD) happen when the impairments of the I/Q branches are correlated or imbalanced, which in turn generate improper additive interference at the receiver side. When the interference is improper, as well as in other interference-limited scenarios, improper Gaussian signaling (IGS) has been shown to provide rate and/or power efficiency benefits. In this paper, we investigate the rate benefits of IGS in a two-user interference channel (IC) with additive asymmetric HWD when interference is treated as noise. We propose two iterative algorithms to optimize the parameters of the improper transmit signals. We first rewrite the rate region as an pseudo-signal-to-interference-plus-noiseratio (PSINR) region and employ majorization minimization and fractional programming to find a suboptimal solution for the achievable user rates. Then, we propose a simplified algorithm based on a separate optimization of the powers and complementary variances of the users, which exhibits lower computational complexity. We show that IGS can improve the performance of the two-user IC with additive HWD. Our proposed algorithms outperform proper Gaussian signaling and competing IGS algorithms in the literature that do not consider asymmetric HWD.},
	doi = {10.1109/TCOMM.2019.2939310},
	author = {Mohammad Soleymani and Lameiro, Christian and Santamar{\'\i}a, Ignacio and Schreier, Peter J.}
}
@article {543,
	title = {Improper Gaussian Signaling for Multiple-Access Channels in Underlay Cognitive Radio},
	journal = {IEEE Transactions on Communications},
	volume = {67},
	year = {2019},
	month = {March},
	pages = {1817-1830},
	abstract = {This  paper considers an unlicensed multiple-accesschannel (MAC) that coexists witha licensed point-to-point user,following  the  underlay  cognitive  radio  paradigm.  We  assumethat  every  transceiver  except  the  secondary  base  station  hasone  antenna  and  that  the  primary  user  (PU)  is  protected  bya   minimum   rate   constraint.  In   contrast   to   the  conventionalassumption of proper Gaussian signaling, we allow the secondaryusers  to  transmit  improper  Gaussian  signals,  which  are  corre-lated  with  their  complex  conjugate.  When  the  secondary  basestation  performs  zero-forcing,  we  show  that  improper  signalingis   optimal   if   the  sum   of   the   interference   channel  gains   (inan  equivalent  canonical  model)  is  above  a  certain  threshold.Additionally,  we  derive  an  efficient  algorithm  to  compute  thetransmission  parameters  that  attain  the  rate  region  boundaryfor  this  scenario.  The  proposed  algorithm  exploits  a  single-userrepresentation of the secondary MAC along with new results onthe optimality of improper signaling in the single-user case whenthe PU is corrupted  by an  improper noise.},
	issn = {1558-0857},
	doi = {10.1109/TCOMM.2018.2880765},
	author = {Lameiro, Christian and Santamar{\'\i}a, Ignacio and Schreier, Peter J.}
}
@article {439,
	title = {Experimental Evaluation of Interference Alignment for Broadband WLAN Systems},
	journal = {EURASIP Journal on Wireless Communications and Networking},
	year = {2015},
	month = {June},
	abstract = {In this paper, we present an experimental study on the performance of spatial interference alignment (IA) in indoor wireless local area network scenarios that use orthogonal frequency division multiplexing (OFDM) according to the physical-layer specifications of the IEEE 802.11a standard. Experiments have been carried out using a wireless network testbed capable of implementing a 3-user MIMO interference channel. We have implemented IA decoding schemes that can be designed according to distinct criteria (e.g., zero-forcing or MaxSINR). The measurement methodology has been validated considering practical issues like the number of OFDM training symbols used for channel estimation or feedback time. In case of asynchronous users, a time-domain IA decoding filter is also compared to its frequency-domain counterpart. We also evaluated the performance of IA from bit error ratio measurement-based results in comparison to different time-division multiple access transmission schemes. The comparison includes single- and multiple-antenna systems transmitting over the dominant mode of the MIMO channel. Our results indicate that spatial IA is suitable for practical indoor scenarios in which wireless channels often exhibit relatively large coherence times.},
	doi = {10.1186/s13638-015-0409-z},
	author = {Lameiro, Christian and Gonz{\'a}lez, {\'O}scar and Garc{\'\i}a-Naya, J. A. and Santamar{\'\i}a, Ignacio and Castedo, Luis}
}
@article {562,
	title = {Ergodic Rate for Fading  Interference Channels with Proper and Improper Gaussian Signaling},
	journal = {Entropy},
	volume = {21},
	year = {2019},
	month = {September},
	doi = {https://doi.org/10.3390/e21100922},
	author = {Mohammad Soleymani and Santamar{\'\i}a, Ignacio and Lameiro, Christian and Schreier, Peter J.}
}
@article {551,
	title = {Design of Asymptotically Optimal Improper Constellations with Hexagonal Packing},
	journal = {IEEE Transactions on Communications},
	volume = {67},
	year = {2019},
	month = {August},
	pages = {5445-5457},
	abstract = {This paper addresses the problem of designing asymptotically optimal improper constellations with a given circularity coefficient (correlation coefficient between the constellation and its complex conjugate). The designed constellations are optimal in the sense that, at high signal-to-noise-ratio (SNR) and for a large number of symbols, yield the lowest probability of error under an average power constraint for additive white Gaussian noise channels. As the number of symbols grows, the optimal constellation is the intersection of the hexagonal lattice with an ellipse whose eccentricity determines the circularity coefficient. Based on this asymptotic result, we propose an algorithm to design finite improper constellations. The proposed constellations provide significant SNR gains with respect to previous improper designs, which were generated through a widely linear transformation of a standard M-ary quadrature amplitude modulation constellation. As an application example, we study the use of these improper constellations by a secondary user in an underlay cognitive radio network.},
	doi = {10.1109/TCOMM.2019.2916857},
	author = {L{\'o}pez-Fern{\'a}ndez, Jes{\'u}s A. and Gonz{\'a}lez Ayestar{\'a}n, Rafael and Santamar{\'\i}a, Ignacio and Lameiro, Christian}
}
@article {412,
	title = {Benefits of Improper Signaling for Underlay Cognitive Radio},
	journal = {IEEE Wireless Communications Letters},
	volume = {4},
	year = {2015},
	month = {Ferbuary},
	pages = {22-25},
	author = {Lameiro, Christian and Santamar{\'\i}a, Ignacio and Schreier, Peter J.}
}
@article {285,
	title = {Amplify-and-Forward Strategies in the Two-Way Relay Channel with Analog Tx-Rx Beamforming},
	journal = {IEEE Transactions on Vehicular Technology},
	volume = {62},
	year = {2013},
	month = {February},
	pages = {642-654},
	author = {Lameiro, Christian and V{\'\i}a, Javier and Santamar{\'\i}a, Ignacio}
}
@conference {390,
	title = {Spatial Shaping and Precoding Design for Underlay MIMO Interference Channels},
	booktitle = {18th International ITG Workshop on Smart Antennas (WSA) },
	year = {2014},
	month = {March},
	address = {Erlangen, Germany},
	author = {Lameiro, Christian and Utschick, Wolfgang and Santamar{\'\i}a, Ignacio}
}
@conference {281,
	title = {Pre- and Post-FFT Interference Leakage Minimization for MIMO OFDM Networks},
	booktitle = {The Ninth International Symposium on Wireless Communication Systems (ISWCS 2012)},
	year = {2012},
	month = {August},
	address = {Paris, France},
	author = {Lameiro, Christian and Gonz{\'a}lez, {\'O}scar and V{\'\i}a, Javier and Santamar{\'\i}a, Ignacio and Heath Jr., Robert W.}
}
@conference {512,
	title = {Performance analysis of maximally improper signaling for multiple-antenna systems},
	booktitle = {IEEE Wireless Communications and Networking Conference (WCNC)},
	year = {2018},
	month = {April},
	address = {Barcelona, Spain},
	author = {Lameiro, Christian and Santamar{\'\i}a, Ignacio and Schreier, Peter J.}
}
@conference {461,
	title = {Maximally Improper Interference in Underlay Cognitive Radio Networks},
	booktitle = {IEEE International Conference on Acoustics, Speech and Signal Processing},
	year = {2016},
	month = {March},
	address = {Shanghai, China},
	author = {Lameiro, Christian and Santamar{\'\i}a, Ignacio and Utschick, Wolfgang and Schreier, Peter J.}
}
@conference {419,
	title = {Interference-Temperature Limit for Cognitive Radio Networks with MIMO Primary Users},
	booktitle = {Asilomar Conference on Signals, Systems, and Computers},
	year = {2014},
	month = {November},
	address = {Pacific Grove, CA, USA},
	author = {Lameiro, Christian and Utschick, Wolfgang and Santamar{\'\i}a, Ignacio}
}
@conference {391,
	title = {Interference Shaping Constraints for Underlay MIMO Interference Channels},
	booktitle = {International Conference on Acoustics, Speech, and Signal Processing (ICASSP)},
	year = {2014},
	month = {May},
	address = {Florence, Italy},
	author = {Lameiro, Christian and Santamar{\'\i}a, Ignacio and Utschick, Wolfgang}
}
@conference {Icassp12_IA_pub,
	title = {Interference Leakage Minimization for Convolutive {MIMO} Interference Channels},
	booktitle = {IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP 2012)},
	year = {2012},
	month = {March},
	address = {Kyoto, Japan},
	author = {Gonz{\'a}lez, {\'O}scar and Lameiro, Christian and V{\'\i}a, Javier and Santamar{\'\i}a, Ignacio and Heath Jr., Robert W.}
}
@conference {349,
	title = {An Interference Alignment Algorithm for Structured Channels},
	booktitle = {IEEE 14th Workshop on Signal Processing Advances in Wireless Communications (SPAWC 2013)},
	year = {2013},
	month = {June},
	address = {Darmstadt, Germany},
	author = {Lameiro, Christian and Gonz{\'a}lez, {\'O}scar and Santamar{\'\i}a, Ignacio}
}
@conference {516,
	title = {Improper Signaling for OFDM Underlay Cognitive Radio Systems},
	booktitle = {IEEE Statistical Signal Processing Workshop (SSP)},
	year = {2018},
	month = {June},
	address = {Freiburg, Germany},
	abstract = {Improper signaling, where real and imaginary parts of the transmit
signal are correlated and/or have unequal power, has received a lot
of attention lately because it has been shown to increase achievable
rates in many interference-limited communication systems. In this paper, we study whether improper signaling can also benefit an orthogonal frequency-division multiplexing (OFDM) underlay cognitive radio (UCR) system. We assume that the primary user (PU) transmits proper signals, while the secondary user (SU) is allowed to employ improper signaling. We consider two different rate constraints for the rate of the PU: i) the total rate of the PU, and ii) the rate of the PU in each subband. We propose an algorithm to implement improper signaling for each constraint. In both cases, we show that the benefits of improper signaling are relatively small and decrease rapidly with increasing number of subbands. This rather negative result shows that the use of improper signaling in interference scenarios needs to be justified on a case-by-case basis.},
	author = {Mohammad Soleymani and Lameiro, Christian and Schreier, Peter J. and Santamar{\'\i}a, Ignacio}
}
@conference {539,
	title = {Improper Gaussian Signaling for the Two-user Broadcast Channel Treating Interference as Noise},
	booktitle = {IEEE Int. Conference on Acoustics, Speech, and Signal Processing (ICASSP)},
	year = {2019},
	month = {May},
	address = {Brighton, UK},
	abstract = {Improper Gaussian signaling (IGS) has been shown to enlarge the rate region achievable by conventional proper Gaussian signaling (PGS) schemes in several interference-limited multiuser networks. In this work, we consider the 2-user broadcast channel (BC) when treating interference as noise {\textquotedblleft}TIN{\textquotedblright} at every receiver. For this scenario, we derive a closed-formcharacterization of the rate region boundary with IGS. ThePareto-optimal points are achieved when at least one of the users employs maximally improper (rectilinear) signals. Differently from other interference-limited networks, our results show that IGS always outperforms PGS for the 2-user BC with TIN. Furthermore, IGS also enlarges the PGS rate region with time-sharing for this scenario.},
	author = {Lameiro, Christian and Santamar{\'\i}a, Ignacio and Schreier, Peter J.}
}
@conference {445,
	title = {An Experimental Evaluation of Broadband Spatial IA for Uncoordinated MIMO-OFDM Systems},
	booktitle = {IEEE International Conference on Digital Signal Processing (DSP)},
	year = {2015},
	month = {July},
	pages = {570--574},
	address = {Singapore},
	author = {Fanjul, Jacobo and Lameiro, Christian and Santamar{\'\i}a, Ignacio and Garc{\'\i}a-Naya, J. A. and Castedo, Luis}
}
@conference {554,
	title = {Energy-Efficient Improper Signaling for K-user Interference Channels},
	booktitle = {European Signal Processing Conference (EUSIPCO)},
	year = {2019},
	month = {September},
	abstract = {This paper investigates the energy efficiency (EE) of improper Gaussian signaling (IGS) in a K-user interference channel (IC). IGS allows unequal variances and/or correlation between the real and imaginary parts, and it has recently been shown to be advantageous in various interference-limited scenarios. In this paper, we propose an energy-efficient IGS design for the K-user IC, which is based on a separate optimization of the powers and complementary variances of the users. We compare the EE region achieved by the proposed scheme with that achieved by conventional proper signaling and show that IGS can significantly improve the EE region.},
	author = {Mohammad Soleymani and Lameiro, Christian and Santamar{\'\i}a, Ignacio and Schreier, Peter J.}
}
@conference {541,
	title = {Energy-Efficient Design for Underlay Cognitive Radio using Improper Signaling},
	booktitle = {IEEE Int. Conference on Acoustics, Speech, and Signal Processing (ICASSP)},
	year = {2019},
	month = {May},
	address = {Brighton, UK},
	abstract = {Improper Gaussian signaling (IGS) has been used as an effective interference management tool in interference limited systems. Improper Gaussian signals are correlated with their complex conjugates. In this paper, we investigate theoptimality of IGS from an energy efficiency (EE) perspective. First, we obtain closed form optimality conditions for
IGS.We then leverage these conditions to devise a bisection method that finds the optimal transmission parameters. Our results show that IGS can improve the EE of an underlay cognitive radio system.},
	author = {Mohammad Soleymani and Lameiro, Christian and Schreier, Peter J. and Santamar{\'\i}a, Ignacio}
}
@conference {276,
	title = {A Distributed Algorithm for Two-Way Multiple-Relay Networks},
	booktitle = {IEEE Sensor Array and Multichannel Signal Processing Workshop (SAM 2012)},
	year = {2012},
	month = {June},
	address = {Hoboken, NJ, USA},
	author = {Lameiro, Christian and V{\'\i}a, Javier and Santamar{\'\i}a, Ignacio}
}
@conference {338,
	title = {Degrees-of-Freedom for the 4-User SISO Interference Channel with Improper Signaling},
	booktitle = {IEEE International Conference on Communications (ICC)},
	year = {2013},
	month = {June},
	address = {Budapest, Hungary},
	author = {Lameiro, Christian and Santamar{\'\i}a, Ignacio}
}
@conference {354,
	title = {Computing the Degrees of Freedom for Arbitrary MIMO Interference Channels},
	booktitle = {IEEE International Conference on Acoustics, Speech, and Signal Processing (ICASSP 2013)},
	year = {2013},
	month = {May},
	address = {Vancouver, Canada},
	abstract = {In this paper we provide an efficient procedure to compute the total number of degrees of freedom (DoF), achievable by linear beamforming, of the K-user multiple-input multiple-output (MIMO) interference channel with an arbitrary number of Tx-Rx antennas at each link. Firstly, we derive an analytical outer bound that generalizes the results that exist for the symmetric K-user M x N interference channel. Secondly, we obtain a tighter bound by solving a convex optimization problem that includes as constraints the DoF characterizations for point-to-point MIMO links and for 2-user interference channels. The solution to this convex problem admits an interesting waterfilling interpretation. Finally, exploiting this outer bound and using a recently proposed feasibility test, we show that it is possible to obtain the DoF for any interference channel in an efficient way. Some simulations results are included to illustrate the tightness of the derived bounds, as well as to study the DoF achievable for the 4-user channel when we distribute the total number of antennas among users and between transmitters and receivers in different ways.},
	keywords = {convex optimization, degrees of freedom, interference alignment, multiple-input multiple-output},
	isbn = {978-1-4799-0356-6},
	author = {Gonz{\'a}lez, {\'O}scar and Lameiro, Christian and V{\'\i}a, Javier and Beltr{\'a}n, Carlos and Santamar{\'\i}a, Ignacio}
}
@conference {Mobiligth2011_-_TWRC-RFAF,
	title = {Capacity Region of the Two-Way Multi-Antenna Relay Channel with Analog Tx-Rx Beamforming},
	booktitle = {3rd International ICST Conference on Mobile Lightweight Wireless Systems (Mobilight 2011)},
	year = {2011},
	month = {May},
	address = {Bilbao, Spain},
	author = {Lameiro, Christian and Naz{\'a}bal, Alfredo and Gholam, Fouad and V{\'\i}a, Javier and Santamar{\'\i}a, Ignacio}
}
@conference {Chinacom2010_MIMOBC,
	title = {Capacity region of the multiantenna Gaussian broadcast channel with analog TX-RX beamforming},
	booktitle = {5th International ICST Conference on Communications and Networking in China (CHINACOM)},
	year = {2010},
	month = {August},
	address = {Beijing, China},
	author = {Santamar{\'\i}a, Ignacio and V{\'\i}a, Javier and Naz{\'a}bal, Alfredo and Lameiro, Christian}
}
@conference {556,
	title = {Benefits of Improper Signaling for Overlay Cognitive Radio},
	booktitle = {IEEE Global Communication Conference (GLOBECOM)},
	year = {2019},
	month = {December},
	address = {Waikoloa, HI, USA},
	abstract = {This paper considers improper Gaussian signaling (IGS) in an overlay cognitive radio scenario. We follow a protocol in which the secondary user (SU) uses part of its power to relay the message for the primary user (PU) and consider a simple yet illustrative 2-user scenario. We analyze two communication schemes depending on whether or not the
PU cooperates with the SU and derive closed-form expressions for the optimal transmission parameters that maximize the SU rate while ensuring a specified minimum performance of the PU. Our numerical results show that IGS may significantly outperform proper signaling and that, interestingly, the cooperative approach provides negligible performance gains over its non-cooperative
counterpart.},
	author = {Lameiro, Christian and Santamar{\'\i}a, Ignacio and Schreier, Peter J.}
}
@conference {430,
	title = {Analysis of maximally improper signalling schemes for underlay cognitive radio},
	booktitle = {IEEE International Conference on Communications (ICC)},
	year = {2015},
	month = {June},
	address = {London, UK},
	author = {Lameiro, Christian and Santamar{\'\i}a, Ignacio and Schreier, Peter J.}
}
