Daniel J. Costello - US grants

A program of basic research is undertaken to explore the use of different coding options on non-uniform channels found in modern digital communication systems. Disturbances such as fading due to multipath transmission or interference cause errors that are received in bursts, resulting in non-uniform statistics for the channel. In this research the performance of codes, both block and convolutional, on non-uniform channels will be investigated. Reasonable channel models will be developed and the performance of different codes for these channel models will be evaluated. Interleaving has long been used in conjunction with coding as a means of achieving reliable communication on burst-error channels. In this research, the performance of interleaving vis a vis burst error correcting codes will be examined. Other important tasks include the investigation of the performance of concatenated coding systems as measured by their effective channel cutoff rate, channel capacity, and undetected error probability.

Trellis-coded modulation (TCM) has proved to be very effective as a modulation technique for bandwidth-efficient communication. Higher data rates over a given channel are obtainable using this technique. Typically, the decoders for this modulation format are Viterbi decoders, which perform a maximum-likelihood sequence estimation upon soft-decision data extracted from the received signal. The complexity of the Viterbi decoders grows exponentially with code constraint length, thus limiting practical operation to moderate coding gains and decoding speeds. The sequential decoding technique gives a performance which is independent of code constraint length thereby permitting the use of longer codes to obtain a greater coding gain. This project is concerned with: 1. The selection of an appropriate sequential decoding algorithm for use with trellis-coed modulation. 2. The performance evaluation of sequential decoding, using both analytical and simulation techniques when used with trellis-coded modulation. 3. The construction of good long-constraint-length trellis codes for use with sequential decoding.

The proposed research plans to improve the performance of trellis coded modulation. Trellis coded modulation (TCM) improves the noise performance of digital transmission without increasing the required bandwidth or reducing the effective data rate. The performance improvement reduces the power requirements for transmission at a specific data rate. %%% The primary goal is to investigate methods of analytically evaluate the distance of TCM codes. In developing a simple, effective method to evaluate TCM codes distances, comparison of codes will be facilitated. In addition, it is hoped that the analytical evaluation method will lead to a code design method which increases TCM code performance. The evaluation technique will be confirmed by computer simulations.

Video display and recording equipment shall be purchased to augment existing research equipment in order to allow simulation of the effects of various algorithms and errors on the quality of digital video sequences under realistic viewing conditions. Particular projects include: * Compression of video data. * Restoration and enhancement of video data through nonlinear and nonstationary filtering. * Visual communication system channel error effect compensation.

Convolutional codes have long been recognized as the preferred method of providing error control over a variety of communication channels. Examples of their practical application can be found in deep space communication, digital satellite transmission, military communications, mobile cellular telephony, and high speed data modems. Despite the many examples of their practical importance, relatively little research has been conducted into the fundamental properties of convolutional codes, compared to other methods of error control. This project is a program of basic research into several new approaches to finding efficient methods of encoding and decoding convolutional codes. Spurred by recent advances in trellis coded modulation, iterative decoding techniques, and the decoding of very complex codes, five areas of research are undertaken: cascaded convolutional codes, multilevel convolutional codes, product convolutional codes, non-standard rate convolutional codes, and unbalanced memory convolutional codes. The goal is to uncover new code structures which can offer better trade-offs between error performance, decoder complexity, and system delay than the standard structure. Particular emphasis is given to studying the fundamental role that delay plays in determining the relationship between performance and complexity. The results of this research may provide viable alternatives to existing coding schemes in applications such as low earth orbiting satellite networks, digital mobile cellular communications, and wireless indoor packet transmissiol systems.

EIA-9977387
Costello, Daniel J.
Collins, Oliver
University of Notre Dame

MRI: Acquisition of Instrumentation for an Experimental Radio Facility in Support of Wireless Digital Communications Research

A team of researchers proposes to construct a facility to provide two-way digital communications capabilities which will consists of two base stations, a number of mobile units and all test equipment necessary to effect narrowband transmission over a geographical area of several square miles. This facility will provide a resource that is rare in academia - an actual operational testbed for research into the algorithms and devices that make wireless digital communications possible. Some of the issues that will be investigated with this facility include: (i) the effectiveness of various error control schemes in a wireless environment, (ii) the design and testing of novel communication devices, circuits and architectures based on compound semiconductor technology, (iii) the ''synergy'' arising from use of spatial diversity in conjunction with coded modulation and interference mitigation, and (iv) experimental testing of proposed new techniques for anticipating the ''deep fades'' that afflict mobile communications.

ABSTRACT

This research involves the general area of error control coding for digital communication and storage systems. In particular, it describes a number of fundamental research topics related to a powerful new method of error control coding called turbo coding. The research has two major goals: (1) to propose new turbo coding schemes with performance and/or complexity advantages compared to the current state-of-the-art, and (2) to advance the fundamental state of knowledge regarding this exciting new approach to error control coding. Although still very new, turbo coding is beginning to be applied in numerous areas that require error control techniques, including deep space communication, satellite communication, and digital cellular telephony, to name just a few. Because of its ability to perform close to theoretical limits with reasonable implementation complexity, it is anticipated that turbo coding and related techniques will have an enormous impact on virtually all applications of error control coding over the next 10 years or so.

Turbo coding can achieve moderate bit error rates (in the range of 10-4 to 10-6) at signal-to-noise ratios very close to channel capacity. However, there is still room for improvement in turbo coding performance, particularly in applications that require bit error rates below 10-6. Further, there is considerable theoretical interest in achieving a more complete fundamental understanding of the key properties of turbo codes that result in such excellent performance. The investigators study several new basic research problems in turbo coding. Among the topics to be investigated are (1) several new turbo code designs capable of achieving even better performance than existing schemes, (2) the introduction of a more general class of turbo codes that has the potential to yield better codes and/or reduced decoding complexity compared to standard turbo coding methods, and (3) the development of a new sub-optimum soft-in, soft-out decoding approach that can be used with more codes, thus offering the promise of near capacity performance at very low bit error rates, say below 10-10.

This award provides funding for a three-year Combined Research-Curriculum Development (CRCD) program, entitled "Real-World Wireless Communications in the Graduate and Undergraduate Curriculum," at the University of Notre Dame, under the direction of Dr. Thomas E. Fuja. The overall objective of this project is to bring elements of the Notre Dame digital communications research and the capabilities created by the wireless facility, the WAND Lab, into the undergraduate and graduate electrical engineering curriculum at Notre Dame.

This project focuses on the physical layer of digital communication
system design - in particular on the design of graph-based error
control coding techniques for noisy, fading channels, such as those
encountered in wireless communication systems. The research
addresses those issues that have made practical implementation of
graph-based coding techniques problematic. Those issues include:

The development of LDPC and turbo codes with algebraic structure,
suitable for high speed implementation.
The development of LDPC and turbo codes with moderate block length,
suitable for practical real-time applications.
The design of bandwidth efficient coded modulation schemes
incorporating these new code constructions in order to "fatten the
bit pipe" in high data rate applications.
The adaptation of these new coding schemes for use in the fading
environment that characterizes wireless channels.

Multihop transmission is increasingly being incorporated into modern wireless communication networks. These networks are central to our nation's future communications and monitoring infrastructures. The
basic motivation for multihop is that transmissions occur over shorter distances -- and therefore with higher received signal strength -- via many intermediate nodes rather than over longer distances -- and
therefore with lower received signal strength -- between the source and destination of the information. However, multihop transmission involves complex interactions among channel coding at the physical
layer, distributed channel access at the link layer, and multihop routing at the network layer. These techniques have been studied largely in isolation by different communities, whereas this project
focuses on their interaction, especially in delay-constrained scenarios.

This research involves models for general wireless multihop networks, and develops tradeoffs for transmission along an individual routes of up to M + 1 nodes. Transmission between the end nodes can occur in a single hop, or up to M hops. Multihop transmission increases the received signal-to-noise ratio (SNR) at intermediate nodes; however, this observation does not take into account the important practical issues of power and bandwidth allocation, end-to-end delay, error propagation, or interference induced by other transmitters. Among other results, preliminary research indicates that the benefits of multihop are eroded by these issues, especially for high spectral efficiency, i.e., high data rates relative to the available bandwidth. The investigators take a comprehensive look at multihop transmission from the point of view of communication theory, mathematical networking, and networking practice, with the goal of offering solutions that will impact a major part of our society.

Collaborative Research:
New Directions in Graph-Based Code Design

Abstract

This collaborative research focuses on the physical layer of digital communication system design ? in particular on the analysis, design, and implementation of capacity-approaching low-density parity-check (LDPC) codes for practical communication environments. In the last ten years, the area of channel coding has undergone a revolutionary change with the growing popularity of graph-based codes and iterative decoding algorithms. These coding methods, which include both turbo codes and LDPC codes, approach the limits of channel coding performance promised by Shannon in his landmark 1948 paper. Currently, these codes are in the process of replacing conventional error control techniques in numerous digital communication and storage standards, including, among others, deep-space communication, next-generation wireless transmission, last-mile cable transmission, digital video broadcasting, and high-density digital magnetic recording.

The research addresses several issues related to graph-based codes. In particular, it focuses on the analysis, design, and implementation of LDPC convolutional codes, which have several advantages compared to LDPC block codes, but have not received much attention from the research community. Conventional convolutional codes, on the other hand, have had a transformative effect in numerous practical communication environments, and the same is likely to be true in the capacity-approaching world of LDPC codes. The project emphasizes bridging the gap between advanced theoretical research and realistic practical implementations. In particular, it is concerned with adapting LDPC convolutional code designs to various industry standards that require flexibility in both frame length and code rate and with developing VLSI implementations of hardware decoders that can be tested under real operating conditions.

Networked wireless communications over multiple hops is rapidly emerging as the main architecture of future wireles systems, including multihop extensions of cellular and WiFi networks, mesh networks, and sensor networks. Common among these types of networks is that they are not completely unstructured (or ad hoc) networks, but traffic is routed and accumulated towards a common destination. Due to this characteristic property, these networks will be referred to as Networks with Traffic Accumulation, or NETAs. Traffic accumulation creates hot sposts or bottlenecks around the common destination because of the increased traffic load and interference. Despite the severity of the hot spot problem, there is a lack of efficient methods to cope with it.

This research addresses the hot spot issue in NETAs by developing new distributed error correction strategies tailored to two important subclasses- line networks and tree networks. In line networks, the investigators study fundamental properties and the design of distributed channel coding protocals using serially concatenated and protograph-based constructions to strengthen the error correction capability near the destination without sacrificing badwidth efficiency. In tree networks, several source nodes may wish to employ a common relay node to broadcast their information to multiple destination nodes which may have access to side information from "overheard" source messages. The investigators explore a novel approach where each source uses a distinctlow rate code for transmission to the relay, whereas the decoded messages are re-encoded using a high rate "nested code". In addition, interlayer issues are considered, in particular the joint design of efficient channel access and routing schemes together with the porposed coding schemes.

This research investigates a new approach to protecting the reliability of digital communication and digital storage systems. This approach takes advantage of recent work (by the research team and others) that formulates the "encoding" and "decoding" of data in terms of a novel graphical representation; this formulation has several advantages over existing techniques for insuring data integrity, including better performance at very low power and the absence of an 'error floor', i.e., the ability to consistently (and significantly) lower the decoded error probability with incremental expenditures of power. The ultimate goal of the research is more reliable delivery of digital data, text, computer files, speech and audio signals, video, etc. - using devices that require less power (and thus have longer battery life) and shorter processing delay.

More specifically, the research investigates the use of spatially coupled sparse codes - channel (error control) codes with a sparse parity check representation formed by coupling together a chain of small "protographs". This approach, which was pioneered by the research team in the context of terminated low-density parity check convolutional codes, has recently been shown to possess a unique combination of properties - iterative decoding performance that approaches channel capacity and minimum distance that grows linearly with block length - as the code size gets large. The research follows four tracks: (1) the design and analysis of low latency/memory decoding strategies; (2) decoded error probability performance guarantees; (3) the development and analysis of spatially coupled sparse codes with algebraic structure; and (4) the application of spatial coupling outside the immediate domain of channel coding, including cooperative diversity, compressed sensing, and multi-terminal source/channel coding.