Codex Corporation

About: Codex Corporation is a based out in . It is known for research contribution in the topics: Signal & Network packet. The organization has 189 authors who have published 241 publications receiving 32205 citations.

Burst-Correcting Codes for the Classic Bursty Channel

Abstract: The purpose of this paper is to organize and clarify the work of the past decade on burst-correcting codes. Our method is, first, to define an idealized model, called the classic bursty channel, toward which most burst-correcting schemes are explicitly or implicitly aimed; next, to bound the best possible performance on this channel; and, finally, to exhibit classes of schemes which are asymptotically optimum and serve as archetypes of the burstcorrecting codes actually in use. In this light we survey and categorize previous work on burst-correcting codes. Finally, we discuss qualitatively the ways in which real channels fail to satisfy the assumptions of the classic bursty channel, and the effects of such failures on the various types of burst-correcting schemes. We conclude by comparing forward-error-correction to the popular alternative of automatic repeat-request (ARQ).

ShuffleNet: an application of generalized perfect shuffles to multihop lightwave networks

Abstract: The authors propose a multihop wavelength-division-multiplexing (WDM) approach, referred to as ShuffleNet, for achieving concurrency in distributed lightwave networks. ShuffleNet can be configured with each user having as few as one fixed-wavelength transmitter and one fixed-wavelength receiver, avoiding both wavelength agility and pretransmission coordination problems. Still, the network can achieve at least 40% of the maximum efficiency possible with wavelength-agile transmitters and receivers. To transmit a packet from one user to another, however, may require routing the packet through intermediate users, each repeating the packet on a new wavelength, until the packet is finally transmitted on a wavelength that the destination user receives. For such a multihop lightwave network, the transmit and receive wavelengths must be assigned to users to provide both a path between all users and the efficient utilization of all wavelength channels. A class of assignment schemes is proposed which is based on a generalization of the perfect shuffle and achieves high efficiency for uniform traffic loads. Physically, the network may take on a variety of topologies, including a bus, tree, or star. >

Convolutional codes II. Maximum-likelihood decoding

Abstract: Convolutional codes are characterized by a trellis structure. Maximum-likelihood decoding is characterized as the finding of the shortest path through the code trellis, an efficient solution for which is the Viterbi algorithm. A universal asymptotic bounding technique is developed and used to bound error probability, free distance, list-of-2 error probability, and other subsidiary quantities. The bounds are dominated by what happens at a certain critical length N erit . Termination of a convolutional code to length N erit or shorter results in an optimum block code. In general, block code exponents can be related to convolutional code exponents and vice versa by a graphical construction, called the concatenation construction. It is shown that termination is unnecessary with the Viterbi algorithm.

Reduced-state sequence estimation for coded modulation of intersymbol interference channels

Abstract: The authors investigated the detection of trellis codes designed for channels that are intersymbol interference free when they operate in the presence of intersymbol interference. A well-structured reduced-state sequence estimation (RSSE) algorithm is described which can achieve the performance of maximum-likelihood sequence estimation (MLSE) with drastically reduced complexity. Well-defined reduced-state trellises are first constructed by merging the states of the ML supertrellis using set partitioning principles. Then the Viterbi algorithm is used to search these trellises. A special case of RSSE, called parallel decision-feedback decoding, uses the encoder trellis, yet on channels with large attenuation distortion it can provide a significantly better performance than linear equalization. The performance of RSSE is examined analytically and through simulation, and then compared to that of MLSE and ideal decision-feedback equalization. It is noted that the performance advantage of RSSE can be obtained without significantly increasing the decoding delay or complicating an adaptive implementation. >

Multidimensional constellations. II. Voronoi constellations

Abstract: For pt.I see ibid., vol.7, no.6, p.877-92 (1989). Voronoi constellations, also called Voronoi codes are implementable N-dimensional constellations based on partitions of N-dimensional lattices ( Lambda ) that can achieve good shape gains and that are inherently suited for use with coded modulation. Two methods are given for specifying Voronoi constellations on the basis of arbitrary lattice partitions Lambda / Lambda /sub s/, where Lambda /sub s/, the shaping lattice, is an N-dimensional sublattice of Lambda . One of the methods is conjectured to be optimum, and the other has desirable symmetries and naturally supports opportunistic secondary channels. When Lambda and Lambda /sub s/ are 2-D-symmetric, the constituent 2-D constellation is itself a Voronoi constellation. The shaping constellation expansion ratio and peak-to-average-power ratio are determined in general and for various Lambda /sub s/. Methods for labeling Voronoi constellations are given. Their complexity is shown to be dominated by that of decoding Lambda /sub s/. It is also shown that coding and shaping are separable and dual. Bounds on the shape gain of Voronoi constellations are given that depend on the depth and normalized informativity of Lambda /sub s/. These bounds suggest the use of lattices Lambda with depth 2 and normalized informativity less than 1, which can achieve near-optimal shape gains with reduced constellation expansion and implementation complexity. >