Institute for Systems Research

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Author: search.filters.author.Fuja, Tom E.Subject: search.filters.subject.distributed information processing

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    Performance Analysis of Coherent TCM Systems with Diversity Reception in Slow Rayleigh Fading
    (1996) Al-Semari, Saud A.; Fuja, Tom E.; ISR
    Coherent trellis coded modulation (TCM) systems employing diversity combining are analyzed. Three different kinds of combining are considered: maximal ratio, equal gain, and selection combining. For each combining scheme, the cutoff rate parameter is derived assuming transmission over a fully- interleaved channel with flat, slow, Rayleigh fading; in addition, tight upper bounds on the pairwise error probabilities are derived. These upper bounds are expressed in product form to permit bounding of the BER via the transfer function approach. In each case it is assumed that the diversity branches are independent and that the channel state information (CSI) can be recovered perfectly.

    Also included is an analysis of maximal ratio combining when the diversity branches are correlated; the cutoff rate and a tight upper bound on the pairwise error probability are derived. It is shown that, with double diversity, a branch correlation coefficient as high as 0.5 results in only slight performance degradation.

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    Optical Orthogonal Codes with Unequal Auto- and Cross- Correlation Constraints
    (1992) Yang, G-C.; Fuja, Tom E.; ISR
    An optical orthogonal code (OOC)is collection of binary sequences with good auto-and cross-correlation properties; they were defined by Salehi and others as a means of obtaining code division multiple access on optical networks. Up to now all work on OOC's have assumed that the constraint placed on the auto- correlation and that placed on the cross-correlation are the same. In this paper we consider codes for which the two constraints are not equal. Specifically, we develop bounds on the size of such OOC's and demonstrate construction techniques for building them. The results demonstrate that a significant increase in the code size is possible by letting the auto- correlation constraint exceed the cross-correlation constraint. These results suggest that for a given performance requirement the optimal OOC may be one with unequal constraints.

    This paper also views OOC's with unequal auto- and cross-correlation constraints as constant-weight unequal error protection (UEP) codes with two levels of protection. The bounds derived are interpreted from this viewpoint and are compared with previous work on UEP codes.

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    Distributed Decoding of Cyclic Block Codes Using a Generalization of Majority-Logic Decoding
    (1992) Murad, A.H.; Fuja, Tom E.; ISR
    One-step majority-logic decoding is one of the simplest algorithms for decoding cyclic block codes. However, it is an effective decoding scheme for very few codes. This paper presents a generalization based on the "common-symbol decoding problem." Suppose one is given M (possibly different) codes over the same field; suppose further that the codewords share a single symbol in common. The common-symbol decoding problem is that of estimating the symbol in the common position. (This is equivalent to one-step majority logic decoding when each of the "constituent" codes is a simple parity check.) This paper formulates conditions under which this decoding is possible and presents a simple algorithm that accomplishes the same. When applied to decoding cyclic block codes, this technique yields a decoder structure ideal for parallel implementation. Furthermore, this approach frequently results in a decoder capable of correcting more errors than one-step majority-logic decoding. To demonstrate the simplicity of the resulting decoders, an example is presented.
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    The Reliability of Systems with Two Levels of Fault Tolerance: The Return of the "Birthday Surprise"
    (1990) Fuja, Tom E.; Yang, G.C.; ISR
    This paper considers the reliability of systems that employ fault tolerance at two different hierarchical levels. Specifically, it assumes the system consists of a two-dimensional array of components. Each component is reliable as long as it has been afflicted by no more than t faults; when t + 1 faults occur in a particular component, the component ceases to be reliable. Furthermore, the system remains operative as long no more than one component in any row is unreliable. By generalizing the techniques used to analyze the well-known "birthday surprise" problem of applied probability, we derive an approximation to the average number of faults needed until the systems fails. Applications include random access memory systems with chip-level and board-level coding as well as fault-tolerant systolic arrays.