Two novel nonlinear companding schemes with iterative receiver to reduce PAPR in multi-carrier modulation systems

Companding transform is an efficient and simple method to reduce the Peak-to-Average Power Ratio (PAPR) for Multi-Carrier Modulation (MCM) systems. But if the MCM signal is only simply operated by inverse companding transform at the receiver, the resultant spectrum may exhibit severe in-band and out-of-band radiation of the distortion components, and considerable peak regrowth by excessive channel noises etc. In order to prevent these problems from occurring, in this paper, two novel nonlinear companding schemes with a iterative receiver are proposed to reduce the PAPR. By transforming the amplitude or power of the original MCM signals into uniform distributed signals, the novel schemes can effectively reduce PAPR for different modulation formats and sub-carrier sizes. Despite moderate complexity increasing at the receiver, but it is especially suitable to be combined with iterative channel estimation. Computer simulation results show that the proposed schemes can offer good system performances without any bandwidth expansion

Constraints on kHz QPO models and stellar EOSs from SAX J1808.4-3658, Cyg X-2 and 4U 1820-30

We test the relativistic precession model (RPM) and the MHD Alfven wave oscillation model (AWOM) for the kHz QPOs by the sources with measured NS masses and twin kHz QPO frequencies. For RPM, the derived NS mass of Cyg X-2 (SAX J1808.4-3658 and 4U 1820-30) is 1.96 +/- 0.10 solar masses (2.83 +/- 0.04 solar masses and 1.85 +/- 0.02 solar masses), which is 30% (100% and 40%) higher than the measured result 1.5 +/- 0.3 solar masses (< 1.4 solar masses and 1.29 + 0.19 / - 0.07 solar masses). For AWOM, where the free parameter of model is the density of star, we infer the NS radii to be around 10 - 20 km for the above three sources, based on which we can infer the matter compositions inside NSs with the help of the equations of state (EOSs). In particular, for SAX J1808.4-3658, AWOM shows a lower mass density of its NS than those of the other known kHz QPO sources, with the radius range of 17 - 20 km, which excludes the strange quark matter inside its star.Comment: 6 pages, 3 figures, 2 table

Assume-guarantee verification for probabilistic systems

We present a compositional verification technique for systems that exhibit both probabilistic and nondeterministic behaviour. We adopt an assume- guarantee approach to verification, where both the assumptions made about system components and the guarantees that they provide are regular safety properties, represented by finite automata. Unlike previous proposals for assume-guarantee reasoning about probabilistic systems, our approach does not require that components interact in a fully synchronous fashion. In addition, the compositional verification method is efficient and fully automated, based on a reduction to the problem of multi-objective probabilistic model checking. We present asymmetric and circular assume-guarantee rules, and show how they can be adapted to form quantitative queries, yielding lower and upper bounds on the actual probabilities that a property is satisfied. Our techniques have been implemented and applied to several large case studies, including instances where conventional probabilistic verification is infeasible

A novel spectral estimation method by using periodic nonuniform sampling

In this paper we present a method of estimating power spectrum density of random ergodic signals. The method allows use of arbitrarily low sampling rates to achieve the goal. We compare our method with similar schemes reported in research literature and argue superiority of our approach in terms of its suitability for practical implementations. The most visible difference between our approach and the previously reported ones consists in replacing Poisson additive random sampling with deterministic sampling. Comparing with the approaches based on the Poisson additive random sampling, where theoreticall infinitely large resources are needed to implement them accurately, our approach clearly relies on limited and well defined resources

Statistical properties of twin kHz QPO in neutron star LMXBs

We collect the data of twin kilohertz quasi-periodic oscillations (kHz QPOs) published before 2012 from 26 neutron star (NS) low-mass X-ray binary (LMXB) sources, then we analyze the centroid frequency (\nu) distribution of twin kHz QPOs (lower frequency \nu_1 and upper frequency \nu_2) both for Atoll and Z sources. For the data without shift-and-add, we find that Atoll and Z sources show the different distributions of \nu_1, \nu_2 and \nu_2/\nu_1, but the same distribution of \Delta\nu (difference of twin kHz QPOs), which indicates that twin kHz QPOs may share the common properties of LXMBs and have the same physical origins. The distribution of \Delta\nu is quite different from constant value, so is \nu_2/\nu_1 from constant ratio. The weighted mean values and maxima of \nu_1 and \nu_2 in Atoll sources are slightly higher than those in Z sources. We also find that shift-and-add technique can reconstruct the distribution of \nu_1 and \Delta\nu. The K-S test results of \nu_1 and \Delta\nu between Atoll and Z sources from data with shift-and-add are quite different from those without it, and we think that this may be caused by the selection biases of the sample. We also study the properties of the quality factor (Q) and the root-mean-squared (rms) amplitude of 4U 0614+09 with the data from the two observational methods, but the errors are too big to make a robust conclusion. The NS spin frequency (\nu_s) distribution of 28 NS-LMXBs show a bigger mean value (about 408Hz) than that (about 281 Hz) of the radio binary millisecond pulsars (MSPs), which may be due to the lack of the spin detections from Z sources (systematically lower than 281 Hz). Furthermore, on the relations between the kHz QPOs and NS spin frequency \nu_s, we find the approximate correlations of the mean values of \Delta\nu with NS spin and its half, respectively.Comment: 10 pages, 5 figures, 5 table

Timescale Spectra in High Energy Astrophysics

A technique of timescale analysis performed directly in the time domain has been developed recently. We have applied the technique to studying rapid variabilities of hard X-rays from neutron star and black hole binaries, gamma-ray bursts and terrestrial gamma-ray flashes. The results indicate that the time domain method of spectral analysis is a powerful tool in revealing the underlying physics in high-energy processes in objects.Comment: 7 pages, 4 figures. Invited talk at the 6th Pacific Rim Conference on Steller Astrophysic

Optimal periodic sampling sequences for nearly-alias-free digital signal processing

Alias-free DSP (DASP) is a methodology of processing signals digitally inside bandwidths that are wider than the famous Nyquist limit of half of the sampling requency. DASP is facilitated by suitable combination of nonuniform sampling and appropriate processing algorithms. In this paper we propose a new method of constructing sampling schemes for the needs of DASP. Unlike traditional approaches that rely on randomly selected sampling instants we use deterministic schemes. A method of optimizing such sequences aimed at minimization of aliasing is proposed. The approach is tested numerically in an experiment where an undersampled signal is processed using DASP; first to estimate the signal's spectrum support function and then the spectrum itself. We demonstrate advantages of the proposed approach over those that use random sampling