| 1. |
- Blom, Rolf
(författare)
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Bounds on key equivocation for simple substitution ciphers
- 1979
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Ingår i: IEEE Transactions on Information Theory. - : IEEE. - 0018-9448 .- 1557-9654. ; 25:1, s. 8-18
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Tidskriftsartikel (refereegranskat)abstract
- The equivocation of the key for a simple substitution cipher is upper and lower hounded, when the message source is memoryless. The hounds are shown to be exponentially tight. The results are compared with random ciphering. It is observed that the exponential behavior of the equivocation of the key is not determined by the redundancy in the message source, but by the symbol probabilities which are closest in a certain sense.
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| 2. |
- Hagander, Per, et al.
(författare)
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A Self-Tuning Filter for Fixed-Lag Smoothing
- 1977
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Ingår i: IEEE Transactions on Information Theory. - 0018-9448. ; 23:3, s. 377-384
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Tidskriftsartikel (refereegranskat)abstract
- The problem of estimating a discrete-time stochastic signal which is corrupted by additive white measurement noise is discussed. How the stationary solution to the fixed-lag smoothing problem can be obtained is shown. The first step is to construct an innovation model for the process. It is then shown how the fixed-lag smoother can be determined from the polynomials in the transfer function of the innovation model. In many applications, the signal model and the characteristics of the noise process are unknown. It is shown that it is possible to derive an algorithm which on-line finds the optimal fixed-lag smoother, a self-tuning smoother. The self-tuning smoother consists of two parts: an on-line estimation of the parameters in the one-step ahead predictor of the measured signal, and a computation of the smoother coefficients by simple manipulation of the predictor parameters. The smoother has good transient, as well as good asymptotic, properties.
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| 3. |
- Johannesson, Rolf, et al.
(författare)
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Further results on binary convolutional codes with an optimum distance profile
- 1978
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Ingår i: IEEE Transactions on Information Theory. - 0018-9448. ; 24:2, s. 264-268
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Tidskriftsartikel (refereegranskat)abstract
- Fixed binary convolutional codes are considered which are simultaneously optimal or near-optimal according to three criteria: namely, distance profiled, free distanced_{ infty}, and minimum number of weightd_{infty}paths. It is shown how the optimum distance profile criterion can be used to limit the search for codes with a large value ofd_{infty}. We present extensive lists of such robustly optimal codes containing rateR = l/2nonsystematic codes, several withd_{infty}superior to that of any previously known code of the same rate and memory; rateR = 2/3systematic codes; and rateR = 2/3nonsystematic codes. As a counterpart to quick-look-in (QLI) codes which are not "transparent," we introduce rateR = 1/2easy-look-in-transparent (ELIT) codes with a feedforward inverse(1 + D,D). In general, ELIT codes haved_{infty}superior to that of QLI codes.
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| 4. |
- Johannesson, Rolf
(författare)
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On the distribution of computation for sequential decoding using the stack algorithm
- 1979
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Ingår i: IEEE Transactions on Information Theory. - 0018-9448. ; 25:3, s. 323-331
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Tidskriftsartikel (refereegranskat)abstract
- An analytical procedure is presented for generating the computational distribution for the Zigangirov-Jelinek stack algorithm. Multitype branching processes are employed to develop a procedure for estimating sequential decoding computation, without the need for simulation, but with sufficient accuracy to be a valid design tool. At information rates about the cutoff rateR_{o}the calculated computational performance is virtually Identical to that obtained by time consuming simulations.
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| 5. |
- Johannesson, Rolf
(författare)
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On the error probability of general trellis codes with applications to sequential decoding
- 1977
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Ingår i: IEEE Transactions on Information Theory. - 0018-9448. ; 23:5, s. 609-611
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Tidskriftsartikel (refereegranskat)abstract
- An upper bound on the average error probability for maximum-likelihood decoding of the ensemble of randomL-branch binary trellis codes of rateR = 1/nis given which separates the effects of the tail lengthTand the memory lengthMof the code. It is shown that the bound is independent of the lengthLof the information Sequence whenM geq T + [nE_{VU}(R)]^{-1} log_{2} L. The implication that the actual error probability behaves similarly is investigated by computer simulations of sequential decoding utilizing the stack algorithm. These simulations confirm the implication which can thus be taken as a design rule for choosingMso that the error probability is reduced to its minimum value for a givenT.
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| 6. |
- Johannesson, Rolf
(författare)
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Robustly-optimal rate one-half binary convolutional codes
- 1975
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Ingår i: IEEE Transactions on Information Theory. - 0018-9448. ; 21:4, s. 464-468
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Tidskriftsartikel (refereegranskat)abstract
- Three optimality criteria for convolutional codes are considered in this correspondence: namely, free distance, minimum distance, and distance profile. Here we report the results of computer searches for rate one-half binary convolutional codes that are "robustly optimal" in the sense of being optimal for one criterion and optimal or near-optimal for the other two criteria. Comparisons with previously known codes are made. The results of a computer simulation are reported to show the importance of the distance profile to computational performance with sequential decoding.
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| 7. |
- Johannesson, Rolf
(författare)
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Some long rate one-half binary convolutional codes with an optimum distance profile
- 1976
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Ingår i: IEEE Transactions on Information Theory. - 0018-9448. ; 22:5, s. 629-631
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Tidskriftsartikel (refereegranskat)abstract
- This correspondence gives a tabulation of long systematic, and long quick-look-in (QLI) nonsystematic, rateR = 1/2binary convolutional codes with an optimum distance profile (ODP). These codes appear attractive for use with sequential decoders.
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| 8. |
- Johannesson, Rolf
(författare)
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Some rate 1/3 and 1/4 binary convolutional codes with an optimum distance profile
- 1977
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Ingår i: IEEE Transactions on Information Theory. - 0018-9448. ; 23:2, s. 281-283
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Tidskriftsartikel (refereegranskat)abstract
- A tabulation of binary systematic convolutional codes with an optimum distance profile for rates1/3and1/4is given. A number of short rate1/3binary nonsystematic convolutional codes are listed. These latter codes are simultaneously optimal for the following distance measures: distance profile, minimum distance, and free distance; they appear attractive for use with Viterbi decoders. Comparisons with previously known codes are made.
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| 9. |
- Kailath, Thomas, et al.
(författare)
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Fast Time-Invariant Implementations of Gaussian Signal Detectors
- 1978
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Ingår i: IEEE Transactions on Information Theory. - : IEEE Information Theory Society. - 0018-9448 .- 1557-9654. ; 24:4, s. 469-477
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Tidskriftsartikel (refereegranskat)abstract
- A new implementation is presented for the optimum likelihood ratio detector for stationary Gaussian signals in white Gaussian noise that uses only two causal time-invariant filters. This solution also has the advantage that fast algorithms based on the Levinson and Chandrasekhar equations can he used for the determination of these time-invariant filters. By using a notion of "closeness to stationarity,' there is a natural extension of the above results for nonstationary signal processes.
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| 10. |
- Ljung, Lennart, 1946-, et al.
(författare)
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Backwards Markovian Models for Second Order Stochastic Processes
- 1976
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Ingår i: IEEE Transactions on Information Theory. - : IEEE Information Theory Society. - 0018-9448 .- 1557-9654. ; 22:4, s. 488-491
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Tidskriftsartikel (refereegranskat)abstract
- A state-space model of a second-order random process is a representation as a linear combination of a set of state-variables which obey first-order linear differential equations driven by an input process that is both white and uncorrelated with the initial values of the state-variables. Such a representation is often called a Markovian representation. There are applications in which it is useful to consider time running backwards and to obtain corresponding backwards Markovian representations. Except in one very special circumstance, these backwards representations cannot be obtained simply by just reversing the direction of time in a forwards Markovian representation. We show how this problem can be solved, give some examples, and also illustrate how the backwards model can be used to clarify certain least squares smoothing formulas.
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