Distance-based kernels for real-valued data

We consider distance-based similarity measures for real-valued vectors of interest in kernel-based machine learning algorithms. In particular, a truncated Euclidean similarity measure and a self-normalized similarity measure related to the Canberra distance. It is proved that they are positive semi-definite (p.s.d.), thus facilitating their use in kernel-based methods, like the Support Vector Machine, a very popular machine learning tool. These kernels may be better suited than standard kernels (like the RBF) in certain situations, that are described in the paper. Some rather general results concerning positivity properties are presented in detail as well as some interesting ways of proving the p.s.d. property.Peer ReviewedPostprint (author's final draft

Piecewise smooth stationary Euler flows with compact support via overdetermined boundary problems

We construct new stationary weak solutions of the 3D Euler equation with compact support. The solutions, which are piecewise smooth and discontinuous across a surface, are axisymmetric with swirl. The range of solutions we find is different from, and larger than, the family of smooth stationary solutions recently obtained by Gavrilov and Constantin-La-Vicol; in particular, these solutions are not localizable. A key step in the proof is the construction of solutions to an overdetermined elliptic boundary value problem where one prescribes both Dirichlet and (nonconstant) Neumann data

Phase only transmit beamforming for spectrum sharing microwave systems

This paper deals with the problem of phase-only transmit beamforming in spectrum sharing microwave systems. In contrast to sub-6 GHz schemes, general microwave systems require a large number of antennas due to its huge path loss. As a consequence, digital beamforming needs a large number of computational resources compared to analog beamforming, which only needs a single radio-frequency chain, results the less computational demanding solution. Analog schemes are usually composed by a phase shifter network whose elements transmit at a certain fixed power so that the system designer shall compute the phase values for each element given a set of directions. This approach leads to non-convex quadratic problems where the traditional semidefinite relaxation fails to deliver satisfactory outcomes. In order to solve this, we propose a nonsmooth method that behaves well in several scenarios. Numerical evaluations in different spectrum sharing scenarios, which show the performance of our method, are provided.Peer ReviewedPostprint (author's final draft

Performance analysis of joint precoding and MUD techniques in multibeam satellite systems

This paper considers interference mitigation techniques in the forward link of multibeam satellite systems. In contrast to previous works, either devoted to receiver interference mitigation (e.g. multiuser detection) or transmitter interference mitigation (precoding), this work evaluates the achievable rates of the joint combination of both techniques. On the one hand, precoding cannot properly mitigate all the inter- beam interference while maintaining a sufficiently high signal-to-noise ratio. On the other hand, the receiver cost and complexity exponentially increases with the number of signals to be simultaneously detected. This highlights that the receiver cannot deal with all the interferences so that in general only 2 signals are jointly detected. As a result, the use of precoding within a coverage area jointly with multiuser detection can both benefit from each other and extremely increase the achievable rates of the system. This is numerically evaluated in a close-to-real coverage area considering simultaneous non-unique decoding strategies. The results show the benefits of this joint scheme that eventually can increase the current precoding performance a 23%.Peer ReviewedPostprint (author's final draft

NOMA and interference limited satellite scenarios

This paper deals with the problem of non-orthogonal multiple access (NOMA) in multibeam satellite systems, where the signals are jointly precoded. It is considered that the number of frames that are simultaneously transmitted is higher than the number of feeds, reducing the precoding interference mitigation capabilities as the system becomes overloaded. In order to solve this problem, we assume that the satellite user terminals are able to perform multi-user detection to mitigate the interference. In the current NOMA approach, it is assumed a successive interference cancellation (SIC) receiver. To increase the spectral efficiency, this paper investigates NOMA with simultaneous non-unique detection (SND). Compared to the case where user terminals perform single user detection (SUD), conventional scheduling heuristic rules do not longer apply in this scenario. Therefore, different scheduling algorithms are proposed considering both SIC and SND strategies. As the numerical evaluations show, SND yields larger average data rates than the SIC receiver. Concerning the scheduling, the best strategy is to pair users with highly correlated channels and the lowest channel gain difference. It is also shown that the sum-rate can be increased in overloaded satellite systems with respect to satellite scenarios, where the number of transmitted frames and feeds is the same.Peer ReviewedPostprint (author's final draft

Memory wars in Russia: the Kremlin versus Memorial

En el presente artículo proponemos un acercamiento a las implicaciones políticas, para el presente y el futuro, de la “elección” de una determinada memoria histórica. Al mismo tiempo, hacemos hincapié en la necesaria vinculación entre los memory studies y los media studies para el análisis riguroso de dichas implicaciones. Desde esta perspectiva teórica, y apoyándonos en conceptos como el de “memoria ejemplar”, analizamos la actividad político-memorialista de la Sociedad “Memorial”, que parte de una mirada a la memoria centrada en dar voz a las víctimas de la represión política, como oposición a la memoria hegemónica en la Rusia de hoy, estructurada entorno al “marco interpretativo de la guerra”.In this paper I intend to probe into the present and future political implications of the ‘choice’ of a specific historical memory, while putting the accent on the necessary link between memory studies and media studies in order to conduct a thorough analysis of them. From this theoretical perspective, and grounded on concepts such as ‘exemplary memory’, what follows is an analysis of the politicalmemory activity of the ‘Memorial’ Society from an approach to memory focused on giving voice to the victims of political repression, in opposition to the hegemonic memory in present-day Russia, structured around the ‘interpretive frame of war’