Bits In Order Research Articles

Separations of non-monotonic randomness notions

In the theory of algorithmic randomness, several notions of random sequence are defined via a game-theoretic approach, and the notions that received the most attention are perhaps Martin-Lof (ML) randomness and computable randomness. The latter notion was introduced by Schnorr and is rather natural: an infinite binary sequence is computably random if no total computable strategy succeeds on it by betting on bits in order. However, computably random sequences can have properties that one may consider to be incompatible with being random, in particular, there are computably random sequences that are highly compressible. The concept of ML randomness is much better behaved in this and other respects, on the other hand its definition in terms of martingales is considerably less natural. Muchnik, elaborating on ideas of Kolmogorov and Loveland, refined Schnorr’s model by also allowing non-monotonic strategies, i.e. strategies that do not bet on bits in order. The subsequent ‘non-monotonic’ notion of randomness, now called Kolmogorov–Loveland randomness, has been shown to be quite close to ML randomness, but whether these two classes coincide remains a fundamental open question. In order to get a better understanding of non-monotonic randomness notions, Miller and Nies introduced some interesting intermediate concepts, where one only allows non-adaptive strategies, i.e. strategies that can still bet non-monotonically, but such that the sequence of betting positions is known in advance (and computable). Recently, these notions were shown by Kastermans and Lempp to differ from ML randomness. We continue the study of the non-monotonic randomness notions introduced by Miller and Nies and obtain results about the Kolmogorov complexities of initial segments that may and may not occur for such sequences, where these results then imply a complete classification of these randomness notions by order of strength.

Relations between threshold and k-interval Boolean functions

Every k-interval Boolean function f can be represented by at most k intervals of integers such that vector x is a truepoint of f if and only if the integer represented by x belongs to one of these k (disjoint) intervals. Since the correspondence of Boolean vectors and integers depends on the order of bits an interval representation is also specified with respect to an order of variables of the represented function. Interval representation can be useful as an efficient representation for special classes of Boolean functions which can be represented by a small number of intervals. In this paper we study inclusion relations between the classes of threshold and k-interval Boolean functions. We show that positive 2-interval functions constitute a (proper) subclass of positive threshold functions and that such inclusion does not hold for any k>2. We also prove that threshold functions do not constitute a subclass of k-interval functions, for any k.

On Guess and Determine Cryptanalysis of LFSR-Based Stream Ciphers

In this paper, the complexity of applying a guess and determine attack to so-called Linear Feedback Shift register (LFSR)-based stream ciphers is analyzed. This family of stream ciphers uses a single or several LFSR and a filtering function F : GF(2) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sup> rarr GF(2) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sup> to generate the blocks of m ges 1 keystream bits at the time. In difference to a classical guess and determine attack, a method based on guessing certain bits in order to determine the remaining secret key/state bits, our approach efficiently takes advantage of the reduced preimage space for relatively large m and at the same time employing the design structure of the cipher. Several variations of the algorithm are derived to circumvent the sensitivity of attack to the input data, n, m and the key length. In certain cases, our attack outperforms classical algebraic attacks; these being considered as one of the most efficient cryptanalyst tools for this type of ciphers. A superior performance of our attack over algebraic attacks is demonstrated in case the filtering function belongs to the extended Maiorana-McFarland class.

Identifying almost identical files using context triggered piecewise hashing

Homologous files share identical sets of bits in the same order. Because such files are not completely identical, traditional techniques such as cryptographic hashing cannot be used to identify them. This paper introduces a new technique for constructing hash signatures by combining a number of traditional hashes whose boundaries are determined by the context of the input. These signatures can be used to identify modified versions of known files even if data has been inserted, modified, or deleted in the new files. The description of this method is followed by a brief analysis of its performance and some sample applications to computer forensics.

Subcarrier allocation and bit loading algorithms for OFDMA-based wireless networks

Orthogonal Frequency Division Multiple Access (OFDMA) is an emerging multiple access technology. In this paper, we consider OFDMA in the context of fixed wireless networks. This paper addresses the problem of assigning subcarriers and bits to point-to-point wireless links in the presence of cochannel interference and Rayleigh fading. The objective is to minimize the total transmitted power over the entire network while satisfying the data rate requirement of each link. We formulate this problem as a constrained optimization problem and present centralized algorithms. The simulation results show that our approach results in an efficient assignment of subcarriers and transmitter power levels in terms of the energy required for transmitting each bit of information. However, centralized algorithms require knowledge of the entire network topology and channel characteristics of every link. In a practical scenario, that would not be the situation and there is a need for distributed rate allocation algorithms. To address this need, we also present a distributed algorithm for allocating subcarriers and bits in order to satisfy the rate requirements of the links.

The Effect of Dose Reduction on Image Quality in Digital Radiography Using a Flat-panel Detector: Experimental Study in Rabbits

Purpose: To evaluate the effect of dose reduction on image quality in digital radiography using a flat-panel detector. Materials and Methods: Digital radiographs of 30 rabbits were obtained at two different dose levels (33.23 Gy for the standard dose group and 20.09 Gy for the reduced dose group). The amorphous selenium-based flatpanel detector system had a panel size of 78.5 inches, a matrix of 12801536 (pixels?), and a pixel pitch of 138 m. Four observers evaluated the soft-copy images on a high-resolution video monitor (256020488 bits) in random order. The observers rated the visibility of 13 different anatomic structures on a 5-point scale, viz. the retrocardiac lung, subdiaphragmatic lung, heart border, diaphragmatic border, proximal airway, unobscured lung, liver border, kidney border, bowel gas, flank stripe, ribs, and vertebrae in the mediastinal and abdominal regions. Statistical significance was determined using Wilcoxon’s signed rank test. Results: There was no statistically significant difference in the visibility of the anatomic structures on digital radiography between the standard and reduced dose groups. Conclusion: Digital radiography using an amorphous selenium-based flat-panel detector can preserve the image quality, even though the does is reduced to 40% of the standard level.

Iterative low-complexity receiver for the UMTS downlink

In this paper, an iterative low-complexity receiver is proposed for Code Division Multiple Access (cdma) systems with small spreading factors. Theumts (Universal Mobile Telecommunication System) radio interface based oncdma has been designed to offer a wide range of data rates using variable spreading factors. High data rate services are obtained by using small spreading factors. For such services, the spreading sequences have bad autocor-relation properties causing the degradation of the Rake receiver performance because of the InterSymbol Interference (isi). In order to improve the receiver performance, we propose to add a Decision Feedback Sequence Estimation (dfse) equalizer at the Rake receiver output. Thedfse is a low complexity equalizer which is able to take into accounta priori probability ratios and to deliver a posteriori probability ratios on bits in order to exchange soft information with the channel decoder, so that the proposed receiver benefits from the turbo-processing gains. Channel estimation is also treated in an iterative fashion. The complete receiver is well suited to theumts downlink system as it drastically reduces theisi while keeping a reasonable computational complexity.

On the Representation of Band Limited Functions Using Finitely Many Bits

In this paper, we consider the question of representing an entire function of finite order and type in terms of finitely many bits, and reconstructing the function from these. Instead of making any further assumptions about the function, we measure the error in reconstruction in a suitably weighted Lp norm. The optimal number of bits in order to obtain a given accuracy is given by the Kolmogorov entropy. We determine this entropy in the case of certain compact subsets of these weighted Lp spaces and obtain constructive algorithms to determine the asymptotically optimal bit representation from finitely many samples of the function. Our theory includes both equidistant and non-uniform sampling. The reconstructions are polynomials, having several other optimality properties.

Optimum bit allocation in subband coding with nonideal reconstruction filters

Subband perceptual audio coding aims at providing the best subjective quality by decomposing the input signal into adjacent subbands. Every frequency component is then separately coded with a different number of bits in order to properly shape the quantization noise density power spectrum. In this letter, an optimum bit allocation algorithm is presented. As well as being applicable to either uniform or nonuniform frequency decompositions, it allows us to take into account the nonideal selectivity of the reconstruction filters. The effects of leakage for both short and long FIR filters have been investigated referring to a wavelet packet subband decomposition.

Mini-fingerprints detect similar activity of receptor ligands previously recognized only by three-dimensional pharmacophore-based methods.

Mini-fingerprints (MFPs) are short binary bit string representations of molecular structure and properties, composed of few selected two-dimensional (2D) descriptors and a number of structural keys. MFPs were specifically designed to recognize compounds with similar activity. Here we report that MFPs are capable of detecting similar activities of some druglike molecules, including endothelin A antagonists and alpha(1)-adrenergic receptor ligands, the recognition of which was previously thought to depend on the use of multiple point three-dimensional (3D) pharmacophore methods. Thus, in these cases, MFPs and pharmacophore fingerprints produce similar results, although they define, in terms of their complexity, opposite ends of the spectrum of methods currently used to study molecular similarity or diversity. For each of the studied compound classes, comparison of MFP bit settings identified a consensus or signature pattern. Scaling factors can be applied to these bits in order to increase the probability of finding compounds with similar activity by virtual screening.

High performance rotation architectures based on the radix-4 CORDIC algorithm

Traditionally, CORDIC algorithms have employed radix-2 in the first n/2 microrotations (n is the precision in bits) in order to preserve a constant scale factor. The authors present a full radix-4 CORDIC algorithm in rotation mode and circular coordinates and its corresponding selection function, and propose an efficient technique for the compensation of the nonconstant scale factor. Three radix-4 CORDIC architectures are implemented: 1) a word serial architecture based on the zero skipping technique, 2) a pipelined architecture, and 3) an application specific architecture (the angles are known beforehand). The first two are general purpose implementations where redundant (carry-save) or nonredundant arithmetic can be used, whereas the last one is a simplification of the first two. The proposed architectures present a good trade-off between latency and hardware complexity when compared with existing CORDIC architectures.

SMILE: A scalable microcontroller library element

A parameterised microcontroller for use in compiled single-chip systems is described. Key processor parameters may be varied over a wide range (minimum data bus width is 4 bits) in order to optimise chip area and performance for a given application. The instruction set is uniform and compact, does not vary when the microcontroller design is scaled, and produces dense code. A working prototype has been produced in Mietec 2.4μ CMOS using the Cathedral silicon compiler, and a variety of 1.2μ and 0.7μ layouts generated from parameterised netlists in the Hilarics and VHDL languages, using both the Cathedral and Synopsys compilers. Code density and layout results are given.

Two new equalisation schemes for high-density digital magnetic recording systems with quad-phase modulation code

Quad-phase has desirable features for use in digital magnetic recording. Here we propose two novel equalisation schemes that seem attractive at high information densities. One scheme estimates the even and odd data bits in reverse order, while the other uses two equalisers to estimate even and odd bits separately.

Long‐range underwater acoustic communciations—A review

Long‐range underwater acoustic communication systems are constrained by the properties of the medium. In particular, the non‐uniform (frequency‐dependent) attenuation of the signal amplitude, coupled with multipath and Doppler‐induced time and frequency spreading, limit long range communications to low frequencies (less than 500 Hz), relatively low data rates (of the order of bits per second), fairly narrow frequency bands. Methods to overcome, or mitigate, the medium‐induced signal degradations are based on attempts to optimize the waveform/receiver structures to the characteristics of the propagation channels. Implicit diversity, provided by convolutional coding and interleaving, frequency‐hopping spread spectrum techniques, and Adaptive RAKE‐like receivers provide considerable signal protection against multipath fading and interference.

Uniaxial Anisotropy ‘Permalloy’ Films grown in a Glow Discharge on Sputtered Tantalum Substrates

IT is well known1 that the magnetization reversal mechanism in a thin ‘Permalloy’ film is related to the degree to which a truly uniaxial anisotropy can be induced on a microscopic scale. Once the magnetostrictive and crystal anisotropy has been minimized by carefully controlling the composition of the film, the surface morphology of the substrate on which the NiFe is to be deposited can play a dominant part in determining Hc and HK and the angular dispersion of the easy axis of magnetization. In magnetic memory applications it is expedient to deposit the ‘Permalloy’ film on to a highly conducting metal ground plane which will be in close proximity to the memory bits in order to reduce both the memory noise and the drive power of the memory array. However, in practice it has not been possible to deposit a thin NiFe film directly on to such a metal ground plane and obtain uniform magnetic properties over large substrate areas, that is, 100 cm2. Magnetic and electron microscopy investigations have shown2 that this is partially due to the fact that mechanical polishing techniques cannot adequately produce the smooth surface characteristics necessary to avoid the inhibition of domain wall motion. In addition, recent work in this laboratory on sputtered NiFe films on very highly polished Ag or Ag.Cu or Au metal planes has shown that significant amounts of the face-centred cubic NiFe diffuses into these face-centred cubic metal substrates during deposition in the glow discharge, resulting in completely isotropic magnetic films. These problems can be overcome by depositing an intermediate layer which is thick enough to serve both as a diffusion barrier which does not replicate the topography of the underlying metal ground plane and at the same time has a minimum of granular structure of its own3. An evaporated SiO film, > 10,000 A, can adequately fulfil these needs2; however, reproducibility over large areas is difficult. The chemistry of SixOy vaporization and the susceptibility of this material to gettering reactions at the substrate with the associated microscopic morphology and anisotropic film stress4 problems all lead to a rather complex system to control accurately. Relatively few alternative thin film materials will adequately fulfil the diffusion barrier and morphology requirements in the deposition temperature-range, 300°–400° C, found necessary5 for uniaxial anisotropy sputtered NiFe films. The possibility of using a thin film of a refractory metal such as tantalum was considered since this would have a low surface mobility on the metal ground plane and would be less susceptible to formation of large granules during the NiFe deposition. Alloying of NiFe with body-centred cubic refractory metals should also not be as serious a problem.