| 41. |
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| 42. |
- Bordenave, Charles, et al.
(författare)
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Asymptotic Stability Region of Slotted Aloha
- 2012
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Ingår i: IEEE Transactions on Information Theory. - 0018-9448 .- 1557-9654. ; 58:9, s. 5841-5855
-
Tidskriftsartikel (refereegranskat)abstract
- We analyze the stability of standard, buffered, slotted-Aloha systems. Specifically, we consider a set of users, each equipped with an infinite buffer. Packets arrive into user i's buffer according to some stationary ergodic Markovian process of intensity lambda(i). At the beginning of each slot, if user i has packets in its buffer, it attempts to transmit a packet with fixed probability p(i) over a shared resource/channel. The transmission is successful only when no other user attempts to use the channel. The stability of such systems has been open since their very first analysis in 1979 by Tsybakov and Mikhailov. In this paper, we propose an approximate stability condition that is provably exact when the number of users grows large. We provide theoretical evidence and numerical experiments to explain why the proposed approximate stability condition is extremely accurate even for systems with a restricted number of users (even two or three).
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| 43. |
- Bouyukliev, I. G., et al.
(författare)
-
Results on Binary Linear Codes With Minimum Distance 8 and 10
- 2011
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Ingår i: IEEE Transactions on Information Theory. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9448 .- 1557-9654. ; 57:9, s. 6089-6093
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Tidskriftsartikel (refereegranskat)abstract
- All linear binary codes with minimum distance 8 and codimension up to 14 and all codes with minimum distance 10 and codimension up to 18 are classified. Nonexistence of codes with parameters [33, 18, 8] and [33, 14, 10] is proved. This leads to 8 new exact bounds for binary linear codes. Primarily two algorithms considering the dual codes are used, namely extension of dual codes with a proper coordinate, and a fast algorithm for finding a maximum clique in a graph, which is modified to find a maximum set of vectors with the right dependency structure.
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| 44. |
- Brännström, Fredrik, 1974, et al.
(författare)
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Classification of Unique Mappings for 8PSK Based on Bit-Wise Distance Spectra
- 2009
-
Ingår i: IEEE Transactions on Information Theory. - 0018-9448 .- 1557-9654. ; 55:3, s. 1131-1145
-
Tidskriftsartikel (refereegranskat)abstract
- The performance of bit-interleaved coded modulation (BICM) with (or without) iterative decoding (ID) is significantly influenced by the mapping of bits to the symbol constellation. Our main objective in this paper is to develop a systematic design approach for BICM-ID schemes, ensuring the best possible performance with iterative decoding. Although useful mappings for BICM-ID have been found based on various search strategies, no attempt has been made to systematically enumerate and classify all unique mappers for a given constellation. As the basis for a systematic enumeration and classification, we define the average bit-wise distance spectrum for a mapping from bits to symbols. Different bit-wise distance spectra are derived assuming no prior information or full prior information, respectively. The bit-wise distance spectra determine corresponding bit-wise error probability and bit-wise mutual information. The latter allows us to use the classification of mappings with unique bit-wise distance spectra to also classify mappings with unique extremal points in the corresponding extrinsic information transfer (EXIT) curves. As an example of our approach, we classify 8PSK mappings into 86 classes of unique mappings according to bit-wise distance spectra. The classification can be used to significantly reduce the complexity of the search for suitable mappers for BICM-ID. For 8PSK and a given encoder, only 86 different mappings need to be, investigated. As examples of the systematic design approach, the best 8PSK mappings for minimizing the convergence threshold are found for concatenation with the rate 1/2 (5, 7)(8) and (133, 171)(8) convolutional codes, and the rate 1/2 UMTS turbo code with identical constituent convolutional codes (15/13)(8).
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| 45. |
- Brännström, Fredrik, 1974, et al.
(författare)
-
Convergence analysis and optimal scheduling for multiple concatenated codes
- 2005
-
Ingår i: IEEE Transactions on Information Theory. - 0018-9448 .- 1557-9654. ; 51:9, s. 3354-3364
-
Tidskriftsartikel (refereegranskat)abstract
- An interesting practical consideration for decoding of serial or parallel concatenated codes with more than two components is the determination of the lowest complexity component decoder schedule which results in convergence. This correspondence presents an algorithm that finds such an optimal decoder schedule. A technique is also given for combining and projecting a series of three-dimensional extrinsic information transfer (EXIT) functions onto a single two-dimensional EXIT chart. This is a useful technique for visualizing the convergence threshold for multiple concatenated codes and provides a design tool for concatenated codes with more than two components.
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| 46. |
- Brännström, Fredrik, 1974, et al.
(författare)
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Optimal Puncturing Ratios and Energy Allocation for Multiple Parallel Concatenated Codes
- 2009
-
Ingår i: IEEE Transactions on Information Theory. - 0018-9448 .- 1557-9654. ; 55:5, s. 2062-2077
-
Tidskriftsartikel (refereegranskat)abstract
- We propose a systematic design framework for optimal, low-complexity punctured multiple parallel concatenated codes (MPCCs), based on minimizing the convergence threshold using extrinsic information transfer (EXIT) charts. As the convergence threshold is related to the area between the two EXIT curves, the corresponding optimization problem is equivalent to a curve-fitting problem. The EXIT curves are determined by the respective EXIT functions of the constituents, which can be conveniently shaped through the use of random puncturing and unequal energy allocations across parallel coding streams. The design task is therefore to find the optimal combination of constituents, puncturing ratios, and energy allocation for matching the EXIT curves. A search over all rate-one convolutional codes of memory length four or less is performed, identifying 98 classes of codes with unique EXIT functions out of a total of 310 codes. Low-complexity MPCCs with up to four constituents are found, where the convergence thresholds are observed to be within 0.1 dB or less of the fundamental minimum signal-to-noise ratio (SNR) corresponding to the binary phase-shift keying (BPSK) capacity for code rates 1/3
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| 47. |
- Bukh, Boris, et al.
(författare)
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An Improved Bound on the Fraction of Correctable Deletions
- 2017
-
Ingår i: IEEE Transactions on Information Theory. - : IEEE. - 0018-9448 .- 1557-9654. ; 63:1, s. 93-103
-
Tidskriftsartikel (refereegranskat)abstract
- We consider codes over fixed alphabets against worst case symbol deletions. For any fixed k >= 2, we construct a family of codes over alphabet of size k with positive rate, which allow efficient recovery from a worst case deletion fraction approaching 1 - (2/(k + root k)). In particular, for binary codes, we are able to recover a fraction of deletions approaching 1/(root 2+1) = root 2-1 approximate to 0.414. Previously, even non-constructively, the largest deletion fraction known to be correctable with positive rate was 1 - Theta (1/root k), and around 0.17 for the binary case. Our result pins down the largest fraction of correctable deletions for k-ary codes as 1 - Theta (1/k), since 1 - 1/k is an upper bound even for the simpler model of erasures where the locations of the missing symbols are known. Closing the gap between (root 2 - 1) and 1/2 for the limit of worst case deletions correctable by binary codes remains a tantalizing open question.
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| 48. |
- Cederlöf, Jörgen, et al.
(författare)
-
Security aspects of the Authentication used in Quantum Cryptography
- 2008
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Ingår i: IEEE Transactions on Information Theory. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9448 .- 1557-9654. ; 54:4, s. 1735-1741
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Tidskriftsartikel (refereegranskat)abstract
- Unconditionally secure message authentication is an important part of Quantum Cryptography (QC). We analyze security effects of using a key obtained from QC for authentication purposes in later rounds of QC. In particular, the eavesdropper gains partial knowledge on the key in QC that may have an effect on the security of the authentication in the later round. Our initial analysis indicates that this partial knowledge has little effect on the authentication part of the system, in agreement with previous results on the issue. However, when taking the full QC protocol into account, the picture is different. By accessing the quantum channel used in QC, the attacker can change the message to be authenticated. This together with partial knowledge of the key does incur a security weakness of the authentication. The underlying reason for this is that the authentication used, which is insensitive to such message changes when the key is unknown, becomes sensitive when used with a partially known key. We suggest a simple solution to this problem, and stress usage of this or an equivalent extra security measure in QC.
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| 49. |
- Cedervall, Mats, et al.
(författare)
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A fast algorithm for computing distance spectrum of convolutional codes
- 1989
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Ingår i: IEEE Transactions on Communications. - : Institute of Electrical and Electronics Engineers (IEEE). - 0090-6778. ; 35:6, s. 1146-1159
-
Tidskriftsartikel (refereegranskat)abstract
- New rate-compatible convolutional (RCC) codes with high constraint lengths and a wide range of code rates are presented. These new codes originate from rate 1/4 optimum distance spectrum (ODS) convolutional parent encoders with constraint lengths 7-10. Low rate encoders (rates 115 down to 1/10) are found by a nested search, and high rate encoders (rates above 1/4) are found by rate-compatible puncturing. The new codes form rate-compatible code families more powerful and flexible than those previously presented. It is shown that these codes are almost as good as the existing optimum convolutional codes of the same fates. The effects of varying the design parameters of the rate-compatible punctured convolutional (RCPC) codes, i.e., the parent encoder rate, the puncturing period, and the constraint length, are also examined. The new codes are then applied to a multicode direct-sequence code-division multiple-access (DS-CDMA) system and are shown to provide good performance and rate-matching capabilities. The results, which are evaluated in terms of the efficiency for Gaussian and Rayleigh fading channels, show that the system efficiency increases with decreasing code rate
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| 50. |
- Cedervall, Mats, et al.
(författare)
-
A new upper bound on the first-event error probability for maximum-likelihood decoding of fixed binary convolutional codes
- 1984
-
Ingår i: IEEE Transactions on Information Theory. - 0018-9448. ; 30:5, s. 762-766
-
Tidskriftsartikel (refereegranskat)abstract
- An upper bound on the first-event error probability for maximum-likelihood decoding of fixed binary convolutional codes on the binary symmetric channel is derived. The bound is evaluated for rate1/2codes, and comparisons are made with simulations and with the bounds of Viterbi, Van de Meeberg, and Post. In particular, the new bound is significantly better than Van de Meeberg's bound for rates above
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