Ana Isabel Pérez Neira

Cross-layer radio resource allocation: The journey so far and the road ahead

The cross-layer concept originated almost ten years ago with the aim of taking the most advantage from the difficult wireless media to break the barriers imposed by the layered transmission. One of the domains where cross-layer design has been more investigated is Radio Resource Allocation, since current and future networks need to provide wireless connectivity to heterogeneous users, offering many different data traffic types. Nonetheless, new paradigms are emerging in the field of wireless communications, like cognitive radios, wireless systems with relays and Multiple Input Multiple Output (MIMO) systems, where the potential advantages of cross-layer scheduling are still largely unknown. Moreover, in spite of a large literature on cross-layer, in the most cases different focuses and perspectives, biased by the application(s), are addressed, thus, there is lack of a general framework. The main goal of this paper is not to provide a state of the art on cross-layer scheduling and resource allocation, since this would result maybe in a heterogeneous list of scientific contributions. Rather, we are interested in performing a cataloguing of the work so far, trying to identify common tools and general frameworks used for cross-layer resource allocation, justify them by means of specific network examples and highlight open issues and challenges to be faced. Nevertheless, a preliminary objective of this work is to introduce a set of definitions which can be hopefully agreed by the scientific community.

Forward link interference mitigation in mobile interactive satellite systems

We present the results of the performance evaluation of polarization-time coding and soft interference cancellation in multi-beam satellite systems a ected by co-channel interference in realistic setups. The standard of Broadband Global Area Network service (BGAN) has been considered as reference for the physical layer and realistic interference and channel models have been adopted. The work has been carried out in the framework of the Next Generation Waveform for Increased Spectral E ciency (NGWISE) project founded by the European Space Agency (ESA).

Low-complexity interference variance estimation methods for coded multicarrier systems: application to SFN

For single-frequency network (SFN) transmission, the echoes coming from different transmitters are superimposed at the reception, giving rise to a frequency selective channel. Although multicarrier modulations lower the dispersion, the demodulated signal is sensitive to be degraded by inter-symbol interference (ISI) and inter-carrier interference (ICI). In view of this, we use channel coding in conjunction either with filter bank multicarrier (FBMC) modulation or with orthogonal frequency division multiplexing (OFDM). To deal with the loss of orthogonality, we have devised an interference-aware receiver that carries out a soft detection under the assumption that the residual interference plus noise (IN) term is Gaussian-distributed. To keep the complexity low, we propose to estimate the variance of the IN term by resorting to data-aided algorithms. Experimental results show that regardless of the method, FBMC provides a slightly better performance in terms of coded bit error rate than OFDM, while the spectral efficiency is increased when FBMC is considered.

Practical implementation of jointly designed transmit receive space-time IIR filters

The study of a joint transmit-receive design over multi-input multi-output (MIMO) channels representing a wireless channel with spatial diversity at both ends is nowadays receiving significant attention. The theoretical closed-form solutions obtained so far are given in the continuous-frequency domain corresponding in general to IIR filters. When those filters are to be implemented in a discrete-time domain, FIR filters are highly desired for many reasons. In a realistic implementation, the filter order is generally constrained to a few taps for computational reasons. We propose practical approximations of the IIR filters using FIR filters constrained to a few taps by efficiently utilizing the degrees of freedom inherent to the filter solutions, i.e., the possibility of freely choosing arbitrary phases appearing in the structure of the filters.

Hybrid analog-digital transmit beamforming for spectrum sharing backhaul networks

This paper deals with the problem of analog–digital transmit beamforming under spectrum sharing constraints for backhaul systems. In contrast to fully digital designs, where the spatial processing is done at baseband unit with all the flexible computational resources of digital processors, analog–digital beamforming schemes require that certain processing is done through analog components, such as phase-shifters or switches. These analog components do not have the same processing flexibility as the digital processor, but on the other hand, they can substantially reduce the cost and complexity of the beamforming solution. This paper presents the joint optimization of the analog and digital parts, which results in a nonconvex, NP-hard, and coupled problem. In order to solve it, an alternating optimization with a penalized convex–concave method is proposed. According to the simulation results, this novel iterative procedure is able to find a solution that behaves close to the fully digital beamforming upper bound scheme.

Spectrum sharing backhaul satellite-terrestrial systems via analog beamforming

Current satellite and terrestrial backhaul systems are deployed in disjoint frequency bands. This fact precludes an efficient use of the spectrum and limits the evolution of wireless backhauling networks. In this paper, we propose an interference mitigation technique in order to allow the spectrum coexistence between satellite and terrestrial backhaul links. This interference reliever is implemented at the terrestrial backhaul nodes, which are assumed to be equipped with multiple antennas. Due to the large bandwidth and huge number of antennas required in these systems, we consider pure analog beamforming. Precisely, we assume a phased array beamforming configuration so that the terrestrial backhaul node can only reduce the interference by changing the phases of each beamforming weight. Two cases are considered: the 18 and 28 GHz band where transmit and receive beamforming optimization problems shall be tackled, respectively. In both cases, the optimization problem results in a nonconvex problem that we propose to solve via two alternative convex approximation methods. These two approaches are evaluated and they present less than 1 dB array gain loss with respect to the upper bound solution. Finally, the spectral efficiency gains of the proposed spectrum sharing scenarios are validated in numerical simulations.

Design of an hybrid ground-space architecture based on individual on-board antenna power optimization for the forward link of a multi-beam broadband satellite system

The multi-beam broadband satellite systems have been deployed to provide broadband services to their under-served geo-graphical area. In one hand, with assuming full frequency reuse among beams the resulting interference requires either a pre-or post-cancellation process. On other hand, by increasing the number of beams, the ground and space units must employ an extensive communication effort on the feeder link (i.e. the link between gateway and satellite) to exchange all of the feed signals, yet maintaining a low processing responsibility to the payload. Since the feeder link spectral resources are scarce, it is desirable to use an on-board beam generation process which leads to a significant feeder link bandwidth reduction. In this context, a hybrid space/ground processing (i.e. both the gateway and the satellite digitally process the transmitted/received data) shall be employed. This work presents a design of the corresponding on-board beam generation process and precoding in the forward link (i.e. from gateway to users in coverage area) of satellite system. In order to effective limit inter-beam interference, this work also predicts a zero forcing precoding technique taking into account the per feed power constraint for practical issues. To this end, we formulate the precoding problem with considering individual on-board feed in order to efficient and more realistic use of payload power resources. This is done by considering the zero forcing and its relation to the theory of generalized inverse in the linear algebra.

Regulation and research on wireless communications

This paper describes wh ich are the major changes would occur in regulation policies and role of the regulation bodies due to the advance of technology development, research and innovation in telecommunications. The so-called open spectrum, instead of free-sp ectrum, seems to offer great opportunities to all curren t players, from operators to technology providers. Open spectrum gives room also to new players mainly for those applications encompassed as short range communications scenarios. This paper remarks that in the vision of future radio co mmunications the regulator, in addition to its current duties, wil l assume an important role on conducting and funding research needed to properly face this new role. Finally an example is provided, which takes advantage of an included reference, in order to put in evidence how many fundamentals of current wireless research will be revisited with unexpected results.

Dynamic bit allocation and differential quantization in feedback link for robust power allocation in multiuser opportunistic beamforming

Multi-antenna systems provide advantages in terms of link quality and multiuser access capabilities, which can be achieved by using schemes as multibeam opportunistic beamforming. This scheme only requires partial channel state information at the transmitter (CSIT) in terms of the equivalent channel moduli. This paper deals with the problem of feeding back the CSI from the multiple receivers to the base station (BS) transmitter through limited capacity feedback links. This involves a quantization and a delay that produces errors in the CSIT. In this sense, the proposed scheme in this paper is based on a robust design that takes into account these errors. Additionally, a dynamic bit allocation in the feedback among the users is derived, and combined with differential quantization to minimize the transmit power. It considers both the userspsila sensitivities to the quantization errors and the Doppler frequencies due to the mobility. Simulation results show the benefits from using such schemes.

Array covariance error measurement in adaptive source estimation

The small error approximation is used to derive a linear relationship between the source parameters (i.e. power levels and directions of arrival) and a measurement of the covariance error matrix, defined as the difference between a nonparametric consistent estimate of the spectral density matrix and a covariance model from the scenario parameters. The resulting framework allows the design of a Kalman-like algorithm which provides a simultaneous and adaptive estimation of the source parameter, no matter what the source waveform or modulation. Good performance is expected, mainly in the presence of sensors malfunctioning, low signal-to-noise ratio, etc.<<ETX>>The small error approximation is used to derive a linear relationship between the source parameters (i.e. power levels and directions of arrival) and a measurement of the covariance error matrix, defined as the difference between a nonparametric consistent estimate of the spectral density matrix and a covariance model from the scenario parameters. The resulting framework allows the design of a Kalman-like algorithm which provides a simultaneous and adaptive estimation of the source parameter, no matter what the source waveform or modulation. Good performance is expected, mainly in the presence of sensors malfunctioning, low signal-to-noise ratio, etc.