Sparse multiple relay selection for network beamforming with individual power constraints using semidefinite relaxation

This paper deals with the multiple relay selection problem in two-hop wireless cooperative networks with individual power constraints at the relays. In particular, it addresses the problem of selecting the best subset of K cooperative nodes and their corresponding beamforming weights so that the signal-to-noise ratio (SNR) is maximized at the destination. This problem is computationally demanding and requires an exhaustive search over all the possible combinations. In order to reduce the complexity, a new suboptimal method is proposed. This technique exhibits a near-optimal performance with a computational burden that is far less than the one needed in the combinatorial search. The proposed method is based on the use of the l1-norm squared and the Charnes-Cooper transformation and naturally leads to a semidefinite programming relaxation with an affordable computational cost. Contrary to other approaches in the literature, the technique exposed herein is based on the knowledge of the second-order statistics of the channels and the relays are not limited to cooperate with full power.Peer ReviewedPostprint (author's final draft

Relay subset selection in cognitive networks with imperfect CSI and individual power constraints

This paper considers the relay subset selection problem in an underlay cognitive network in which two secondary users communicate assisted by a set of N potential relays. More specifically, this paper deals with the joint problem of choosing the best subset of L secondary relays and their corresponding weights which maximize the Signal-to-Interference-plus-Noise ratio (SINR) at the secondary user receiver, subject to per-relay power constraints and interference power constraints at the primary user. This problem is a combinatorial problem with a high computational burden. Nevertheless, we propose a sub-optimal technique, based on a convex relaxation of the problem, which achieves a near-optimal performance with a reduced complexity. Contrary to other approaches in the literature, the secondary relays are not limited to cooperate at full power.Peer ReviewedPostprint (author's final draft

Conformador de haz de referencia por código de dos etapas: aplicación a la recepción de señales con modulación de saltos frecuenciales

La tesis propone un nuevo conformador de haz para la recepción de señales con modulación de saltos frecuenciales "frequency hopping" (FH) basado en el conocimiento de la secuencia de frecuencias utilizada en la modulación denominándose por ello conformador de haz de referencia por código "code reference beamformer" (CRB). Este conformador se divide en dos etapas, cada una de ellas implementa uno de los factores del vector de enfoque óptimo; es decir, la primera etapa es igual a la inversa de la matriz de covarianza de ruido más interferencias y la segunda es igual al vector de dirección de la señal deseada. La obtención de estas etapas es posible gracias a la predicción del escenario que se tendrá durante el salto siguiente, la cual puede realizarse simultáneamente a la recepción gracias al conocimiento de la secuencia frecuencial. El sistema receptor se compone de dos procesadores: el procesador anticipado que se dedica a la estimación de la matriz de covarianza de ruido más interferencias presentes en la banda frecuencial del siguiente salto y el procesador directo que es el encargado de la conformación de haz. Finalmente se propone la extensión de crb a un conformador de haz autocalibrado de aplicación general que no requiere referencia alguna, ni temporal ni espacial.Postprint (published version

High resolution adaptive arrays based on random processing techniques: frequency hopping modulation

A new architecture for adaptive arrays using frequency hopping modulation is addressed. The resolution of the array and the interference rejection increase substantially applying random processing to the carrier frequency of the signals. The proposed framework is composed of two different stages. The anticipative stage, devoted to minimize the noise and fixed interferences contribution and the GSLC stage which provides cancellation of follower jammers and solves the multiuser collision problem. The developed system requires neither temporal nor spatial reference for its implementation, only the frequency sequence must be known. An adaptive approach has been implemented, allowing a fast convergence to the optimal behavior.Peer ReviewedPostprint (published version

Adaptive array beamforming for frequency hopping modulation

A new architecture for Array Processing using Frequency Hopping (FH) modulation is addressed in this paper which takes advantage of the knowledge of the frequency sequence at the receiver, requiring neither temporal nor spatial a priori reference. Consequently, the paper deals with a Code Reference Beamformer (CRB). The proposed framework is composed of two parallel processors. The first one, the Anticipative processor, is devoted to predict the scenario at the hop frequency before this frequency is transmitted, providing a fast convergence of the second processor and avoiding the fall of the Signal to Interference plus Noise Ratio (SINR) with the frequency hops. The second one, the On-line processor, provides maximum SINR by applying the optimum beamvector which can be estimated minimizing the Mean Square Error (MSE) at the array output or, directly, maximizing the SINR.Peer ReviewedPostprint (published version

Pointing correction in an adaptive array for communications

The authors discuss DSP procedures for pointing correction in adaptive arrays which uses a spatial reference to form the optimum beam (generalized sidelobe canceller, GSLC). The problem of adequate pointing arises from many effects on the aperture and the receiver front-end of every array channel. Component aging, channel mismatching, sensor location, etc., promote steering errors that may produce the rejection of the desired signal. Currently, highly complicated telemetry systems and monopulse radars are used for steering correction. The authors describe the existing alternatives as well as further improvements that can be achieved from the use of temporal references combined with the GSLC beamformerPeer ReviewedPostprint (published version

Joint beamforming and channel estimation for pilot-aided WCDMA systems

The problem of joint beamforming and channel estimation for multi-rate multi-code systems is addressed. Usual schemes perform this filtering/estimation operation making use of a training sequence time-multiplexed with the transmitted data. However if pilot and traffic signals are transmitted simultaneously using distinct code allocation-as it is the case in recent standards such as cdma2000 or WCDMA-these schemes tend to fail. This paper proposes semi-blind techniques to overcome the uplink auto-interfering effects of such systems. It is shown that the semi-blind approach yields substantially better performance results thanks to the implicit modeling of the unknown traffic data.Peer ReviewedPostprint (published version

Double shrinkage correction in sample LMMSE estimation

The sample linear minimum mean square error (LMMSE) es- timator undergoes high performance degradation in the small sample size regime. Herein a double shrinkage correction is proposed to alleviate this problem. First, an af ne transfor- mation of the sample covariance matrix (SCM) is considered within the LMMSE. Second, a linear transformation of that modi ed lter is proposed. The linear transformation mini- mizes the asymptotic MSE of the lter given a shrinkage of the SCM. And the shrinkage of the SCM optimizes the as- ymptotic MSE of the data covariance. Simulations highlight that the proposed estimator outperforms robust methods to the small sample size, namely LMMSE based on diagonal load- ing (DL) or Ledoit-Wolf (LW) regularizations of the SCMPeer ReviewedPostprint (published version

Adaptive Beamforming for High Bit Rate Services in the FDD Mode of UTRA

Most time-reference beamforming algorithms suffer from severe beampattern distortion effects when applied to high bit rate services in WCDMA, causing serious performance degradation in terms of output BER, especially at high input SINR levels. These shortcomings are essentially caused by the uplink multiplexing of the traffic channel, which is seen by the base station as a powerful interfering source coming from the direction of arrival of the desired user. In this paper, a semi-blind beamforming technique is proposed as a valid solution to overcome this effect. The suggested scheme resorts to a conditional maximum likelihood approach to the underlying estimation problem and is designed to operate in an iterative fashion.Peer ReviewedPostprint (published version

Performance comparison between FBMC and OFDM in MIMO systems under channel uncertainty

Postprint (published version