Browsing by Subject "Block codes"

Now showing 1 - 7 of 7

Open Access
Guessing subject to distortion
(Institute of Electrical and Electronics Engineers, 1998-05) Arikan, E.; Merhav, N.
We investigate the problem of guessing a random vector X within distortion level D. Our aim is to characterize the best attainable performance in the sense of minimizing, in some probabilistic sense, the number of required guesses G(X) until the error falls below D. The underlying motivation is that G(X) is the number of candidate codewords to be examined by a rate-distortion block encoder until a satisfactory codeword is found. In particular, for memoryless sources, we provide a single-letter characterization of the least achievable exponential growth rate of the ρth moment of G(X) as the dimension of the random vector X grows without bound. In this context, we propose an asymptotically optimal guessing scheme that is universal both with respect to the information source and the value of ρ. We then study some properties of the exponent function E(D, ρ) along with its relation to the source-coding exponents. Finally, we provide extensions of our main results to the Gaussian case, guessing with side information, and sources with memory.
Open Access
Improved DST cryptanalysis of IDEA
(Springer, 2006-08) Ayaz, Eyüp Serdar; Selçuk, Ali Aydın
In this paper, we show how the Demirci-Selcuk-Ture attack, which is currently the deepest penetrating attack on the IDEA block cipher, can be improved significantly in performance. The improvements presented reduce the attack's plaintext, memory, precomputation time, and key search time complexities. These improvements also make a practical implementation of the attack on reduced versions of IDEA possible, enabling the first experimental verifications of the DST attack. © Springer-Verlag Berlin Heidelberg 2007.
Open Access
A low-complexity time-domain MMSE channel estimator for space-time/frequency block-coded OFDM systems
(Hindawi Publishing Corporation, 2006) Şenol, H.; Çırpan, H. A.; Panayırcı, E.; Çevik, M.
Focusing on transmit diversity orthogonal frequency-division multiplexing (OFDM) transmission through frequency-selective channels, this paper pursues a channel estimation approach in time domain for both space-frequency OFDM (SF-OFDM) and space-time OFDM(ST-OFDM) systems based on AR channel modelling. The paper proposes a computationally efficient, pilot-aided linear minimum mean-square-error (MMSE) time-domain channel estimation algorithm for OFDM systems with transmitter diversity in unknown wireless fading channels. The proposed approach employs a convenient representation of the channel impulse responses based on the Karhunen-Loeve (KL) orthogonal expansion and finds MMSE estimates of the uncorrelated KL series expansion coefficients. Based on such an expansion, no matrix inversion is required in the proposed MMSE estimator. Subsequently, optimal rank reduction is applied to obtain significant taps resulting in a smaller computational load on the proposed estimation algorithm. The performance of the proposed approach is studied through the analytical results and computer simulations. In order to explore the performance, the closed-form expression for the average symbol error rate (SER)probability is derived for the maximum ratio receive combiner(MRRC). We then consider the stochastic Cramer-Rao lower bound(CRLB) and derive the closed-form expression for the random KL coefficients, and consequently exploit the performance of the MMSE channel estimator based on the evaluation of minimum Bayesian MSE. We also analyze the effect of a modelling mismatch on the estimator performance. Simulation results confirm our theoretical analysis and illustrate that the proposed algorithms are capable of tracking fast fading and improving overall performance. Copyright © 2006 Hindawi Publishing Corporation. All rights reserved.
Open Access
Lower bounds to moments of list size
(IEEE, 1990) Arıkan, Erdal
Summary form only given. The list-size random variable L for a block code is defined as the number of incorrect messages that appear to a maximum-likelihood decoder to be at least as likely as the true message. Lower bounds to the moments of L have been obtained. For sequential decoding, the results imply that the tth moment of computation is unbounded at rates above a certain value, for all t≥0, and settle a long-standing open problem.
Open Access
Multiresolution block coding method for visualization of compressed images in multimedia applications
(Society of Photo-Optical Instrumentation Engineers, 1994) Gerek, O. N.; Cetin, E. A.
Multimedia and Picture Archiving and Communication System (PACS) applications require efficient ways of handling images for communication and visualization. In many Visual Information and Management Systems (VIMS), it may be required to get quick responses to queries. Usually, a VIMS database has a huge number of images and may provide lots of images for each query. For example, in a PACS, the VIMS provides 10 to 100 images for a typical query. Only a few of these images may actually be needed. In order to find the useful ones, the user has to preview each image by fully decompressing it. This is neither computationally efficient, nor user friendly. In this paper, we propose a scheme which provides a magnifying glass type previewing feature. With this method, a multiresolution previewing without decompressing the whole image is possible. Our scheme is based on block transform coding which is the most widely used technique in image and video coding. In the first step of our scheme, all of the queried images are displayed in the lowest possible resolution (constructed from the DC coefficients of the coded blocks). If the user requests more information for a region of a particular image by specifying its size and place, then that region is hierarchically decompressed and displayed. In this way, large amounts of computations and bandwidth usage are avoided and a good user interface is accomplished. This method changes the ordering strategy of transform coefficients, thus reduces the compression ratio, however this effect is small.
Open Access
On probability of success in linear and differential cryptanalysis
(Springer New York LLC, 2008-01) Selçuk, A. A.
Despite their widespread usage in block cipher security, linear and differential cryptanalysis still lack a robust treatment of their success probability, and the success chances of these attacks have commonly been estimated in a rather ad hoc fashion. In this paper, we present an analytical calculation of the success probability of linear and differential cryptanalytic attacks. The results apply to an extended sense of the term "success" where the correct key is found not necessarily as the highest-ranking candidate but within a set of high-ranking candidates. Experimental results show that the analysis provides accurate results in most cases, especially in linear cryptanalysis. In cases where the results are less accurate, as in certain cases of differential cryptanalysis, the results are useful to provide approximate estimates of the success probability and the necessary plaintext requirement. The analysis also reveals that the attacked key length in differential cryptanalysis is one of the factors that affect the success probability directly besides the signal-to-noise ratio and the available plaintext amount. © 2007 International Association for Cryptologic Research.
Open Access
Turbo/katlamalı kodlanmış SFBC/STBC-OFDM sistemlerinde yinelemeli kanal kestirimi
(2006-04) Doǧan, H.; Çırpan, H. A.; Panayırcı, Erdal
We consider the design of turbo receiver structures for space-frequency block coded (SFBC/STBC) OFDM systems in unknown frequency selective fading channels. The Turbo receiver structures for SFBC/STBC-OFDM systems under consideration consists of an iterative MAP Expectation/Maximization (EM) channel estimation algorithm, soft MMSE SFBC/STBC decoder and a soft MAP outer-channel-code decoder. MAP-EM employs iterative channel estimation and it improves receiver performance by re-estimating the channel after each decoder iteration. Moreover, the MAP-EM approach considers the channel variations as random processes and applies the Karhunen-Loeve (KL) orthogonal series expansion. The performance of the proposed approaches are studied in terms of mean square error and bit-error rate. Through experimental results, the effect of a pilot spacing on the channel estimator performance and sensitivity of turbo receiver structures on channel estimation error are studied. © 2006 IEEE.