Antenna phase pattern measurements at millimeter wave frequencies using the differential phase method with only one power meter

The differential phase measurement method has been improved to need the use of only one power meter instead of three power meters. This enables accurate antenna phase pattern measurements with a simplified set-up, accompanied by the reduction in cost. All advantages of the differential phase measurement method are still also available, e.g. there is no need to phase lock oscillators or to use rotary joints. The measurement results for an antenna at 110 GHz are presented. A good agreement with earlier data was obtained.     

Evidence for a surface state with broken symmetry in3He-B from ROTA experiments at Helsinki

An unusual, newly observed, rotation-direction-dependent parity effect in nuclear magnetic resonance (NMR) experiments on rotating³He-B in a state void of vortices is here first explained in terms of the broken time-inversion symmetry (T) for the surface layer of the³He-B. Two possible types of such surface structures, both with brokenT symmetry, for superfluid³He-B near the wall of the container are consistent with properties of the parity effect: one has superfluid A phase on the³He-B surface, while the other one is characterized by a nonzero counterflow of the spin componentsS=0 andS=+-1 of the superfluid along the normal to the wall, but with the total mass supercurrent and the total spin supercurrent into the wall vanishing identically. The first surface structure is in better agreement with details of the experiment and our numerical results, which demonstrate, for the first time, absolute stability of an A-phase layer on the³He-B boundary in a certain region of the phenomenological parameters for the surface tension. We find it also possible for a first-order surface-wetting transition to occur by the A-phase layer between two different A-phase states: one with little A phase, the other with well developed A phase. We estimate the magnitude of the new surface-orientating effects by comparing the NMR data with computed spin-wave spectra. New types of point vortices on the surface, boojums, are possible for the³He-B with superfluid A phase on the boundary: unlike the usual boojums on the surface of³He-A, only these novel boojums on the³He-B surface display isolated half-integer quanta of superfluid circulation.     

Half-solitons in superfluid3He-A: Novel π/2-quanta of phase slippage

Core structures of nontopological solitons between inequivalent vacua in superfluid³He-A are considered. We analyze the symmetries of these A-A interfaces, and compute their hard-core structures in the Ginzburg-Landau regime. We discuss both domain walls where the orbital anisotropy l-vector is flipped (l–l), and those with the same l(x=–) and l(x=+) asymptotics. In particular, we find new classes of A-A boundaries: these novel /2-solitons, which can occur in the absence of a change in the asymptotic l-vector field, constitute the elementary quanta of phase slippage in superfluid³He-A. We ascribe these half-solitons to a new topological scenario for the flaring-out of vorticity in the extended (k, r)-space. Edges of such walls serve to provide vortices with 1/4 quantum of circulation in³He-A. In analogy with the B-B domain walls in superfluid³He-B, solitons of pure phase slippage by —with a normal core—prove unstable in³He-A; they either fission into a pair of ordinary l-solitons—domain walls with superfluid cores both flipping the orbital anisotropy axis (i.e., l–ll), or form a bound pair of walls, each of which constitutes an l-soliton with a phase shift of /2. Our investigation of the superfluid A-A vacuum interfaces may prove useful in a broader context since the A-A boundaries exemplify the possible domain walls relevant for the Higgs-field solitons (cosmic domain walls) within the Weinberg-Salam model.     

Cellular automaton-based algorithms for the dispersion of mobile wireless sensor networks

Due to the advent of sensor technology and its applications, mobile wireless sensor networks (MWSNs) have gained a significant amount of research interest. In a typical MWSN, sensors can move within the network. We develop a set of probabilistic and deterministic cellular automaton (CA)-based algorithms for motion planning problems in MWSNs. First, we consider a scenario where a group of sensors are deployed and they need to disperse in order to maximise the area covered by the network. In this variant of the problem we do not explicitly consider that the sensors should maintain the connectivity of the network while they move. Second, we consider a scenario where the sensors are initially randomly distributed and they need to disperse autonomously to both maximise the coverage of the network and maintain its connectivity. We carry out extensive simulations of both deterministic and randomised variants of the algorithms. For the first variant of the problem we compare our algorithms with one previous algorithm and find that our algorithm yields better network coverage than the earlier algorithm. We also find that probabilistic algorithms have better overall performance for the second variant. CA algorithms rely only on local information about the network and, hence, they can be used in practice for MWSN problems. On the other hand, locality of the algorithm implies that maintaining connectivity becomes a non-trivial problem.     

Pseudo-inversion: closure properties and decidability

We consider a pseudo-inversion operation inspired by biological events, such as DNA sequence transformations, where only parts of a string are reversed. We define the pseudo-inversion of a string \(w = uxv\) to be the set of all strings \(v^Rxu^R\), where \(uv \ne \lambda \) and consider the operation from a formal language theoretic viewpoint. We show that regular languages are closed under the pseudo-inversion operation whereas context-free languages are not. Furthermore, we study the iterated pseudo-inversion operation and show that the iterated pseudo-inversion of a context-free language is recognized by a nondeterministic reversal-bounded multicounter machine. Finally, we introduce the notion of pseudo-inversion-freeness and examine closure properties and decidability problems for regular and context-free languages. We demonstrate that pseudo-inversion-freeness is decidable in polynomial time for regular languages and undecidable for context-free languages.     

Carousel interferometer

A new type of a swinging interferometer, the carousel interferometer, is presented, and its properties are studied and compared with other swinging interferometers. The new interferometer is built with five plane mirrors. The optical path difference is accomplished by rotation of a system that consists of four mirrors. The modulation is almost independent of the scanning of the interferometer. It is not sensitive to external perturbations such as bending or other deformations of the mount. The construction is very compact. Because of its stability and low cost it is very applicable to small Fourier-transform spectrometers for any wave-number region from the far infrared down to the ultraviolet.     

Infinite factorization of multiple non-parametric views

Combined analysis of multiple data sources has increasing application interest, in particular for distinguishing shared and source-specific aspects. We extend this rationale to the generative and non-parametric clustering setting by introducing a novel non-parametric hierarchical mixture model. The lower level of the model describes each source with a flexible non-parametric mixture, and the top level combines these to describe commonalities of the sources. The lower-level clusters arise from hierarchical Dirichlet Processes, inducing an infinite-dimensional contingency table between the sources. The commonalities between the sources are modeled by an infinite component model of the contingency table, interpretable as non-negative factorization of infinite matrices, or as a prior for infinite contingency tables. With Gaussian mixture components plugged in for continuous measurements, the model is applied to two views of genes, mRNA expression and abundance of the produced proteins, to expose groups of genes that are co-regulated in either or both of the views. We discover complex relationships between the marginals (that are multimodal in both marginals) that would remain undetected by simpler models. Cluster analysis of co-expression is a standard method of screening for co-regulation, and the two-view analysis extends the approach to distinguishing between pre- and post-translational regulation.     

State complexity of basic operations on suffix-free regular languages

We investigate the state complexity of basic operations for suffix-free regular languages. The state complexity of an operation for regular languages is the number of states that are necessary and sufficient in the worst-case for the minimal deterministic finite-state automaton that accepts the language obtained from the operation. We establish the precise state complexity of catenation, Kleene star, reversal and the Boolean operations for suffix-free regular languages.