Showing papers in "IEEE Journal on Selected Areas in Communications in 1985"

Pattern Code Modulation and Optical Decoding - A Novel Code-Division Multiplexing Technique for Multifiber Networks

Abstract: We consider the use of multiple high-capacity fibers for communications networking. Each user transmits, asynchronously, patterns of optical pulses distributed over the fibers and throughout a time frame. Each receiver has a distinct alphabet of patterns, which are detected by optical correlators. Optical correlation by fiber tapped delay lines provides speedy and easy-to-implement decoders. Thus the individual user obtains a transparent low-speed channel by code multiplexing. The reliability of this low-speed channel can be enhanced by redundantly coding the patterns sent by the user, for which the encoding and decoding processes can be performed electronically. This two-step encoding process is simple to implement, highly reliable at reasonable throughput, and provides asynchronous access with simple protocol. Various components and configurations of this access scheme are described. The information theoretic capacity and the error probability for these configurations are derived. We also demonstrate that hardlimiting and filtering at the receiver reduce error probability significantly.

SNA Networks of Small Systems

Abstract: This paper discusses SNA/LEN, a possible extension of Systems Network Architecture intended to allow peer, dynamic, and easy to use networking functions for a variety of node sizes down to and including the new generation of personal computers. After reviewing the special requirements posed by small systems and the need for including them as equal partners in networks, the LEN architecture is described. This is done by following a sequence of steps that begin when a logical resource at some node that may not yet be part of the connectivity of an existing network requests a session with a remote logical resource of unknown location. After connectivity with the preexisiting network is established, directory services locates the remote object, route selection services determines the preferred path, a session is activated, and deadlock-free flow control assures a useful flow of data. Preliminary quantitative results from an LEN prototype are presented.

Packet Radio in the Amateur Service

Abstract: During the past several years there has been much activity in the development of networking hardware and software for personal computers. At the same time the amateur radio community has been equally active in the development of networking equipment and procedures especially suited for radio circuits. Although the initial efforts have been with terrestrial circuits, experimentation with and development of satellite-based systems is well underway. While we are aware of the many commercial and government organizations active in specialized packet radio systems, our goal is to establish radio networks with inexpensive equipment that is within reach of a large number of individuals with limited budgets. This article will provide a brief history of hardware, software, and protocol development activities as well as a description of amateur packet radio operations. Also discussed are satellite-based operations and development efforts. These include AMSAT-OSCAR-10 and UoSAT-OSCAR-11, currently operating in orbit, and PACSAT, JAS-1, and Phase 3-C, in the design and construction phases.

The Application of Digital Broadcast Communication to Large Scale Information Systems

Abstract: A new type of information system is described that combines personal computers, broadcast data communication, and bidirectional communication. The system is designed to use broadcast communication whenever possible to deliver information to personal computers, which are used for data storage, indexing, and retrieval. This paper starts with an overview of the system, and then discuss the problem of reliable digital broadcast communication in some detail. A parameterized broadcast protocol is described, and we show how to choose protocol parameters based on observed channel error characteristics. A flexible encryption-based protection system is included in the protocol. We discuss the implementation of the system on contemporary personal computers. A broadcast system based on these ideas is now operating in Boston area homes.

LPI Detection of Frequency-Hopping Signals Using Autocorrelation Techniques

Abstract: Interception of frequency-hopping (FH) waveforms is commonly achieved by using a set of radiometers (energy detectors) that individually energy-detect subbands of the total spread bandwidth of the suspected transmission. In this paper, a scheme is analyzed which can improve the detection performance of FH waveforms in wide-band additive Gaussian noise (AWGN) using samples from the autocorrelation domain. It is shown that, under fairly general operational assumptions, an appreciable gain in decision signal-to-noise ratio (SNR) can be achieved over that of the radiometer. This gain is proportional to \gamma_{H}^{2} where \gamma _{H} is the hop SNR. The proposed algorithm, albeit inferior to the optimal likelihood-ratio test, has the advantage of reduced complexity. The overall approach has been motivated by the recent implementational feasibility of large time-bandwidth-product real-time correlators such as surface-acoustic wave devices (SAWD's).

Performance of Noncoherent Direct-Sequence Spread-Spectrum Multiple-Access Communications

Abstract: The performance of noncoherent reception in direct-sequence spread-spectrum multiple-access communications systems is investigated for additive white Gaussian noise channels. Analytical and numerical results on the probability of error are presented for binary and M -ary frequency-shift-keying data modulation with noncoherent demodulation and differential-phase-shift-keying data modulation with differentiallycoherent demodulation. Both synchronous and asynchronous systems are analyzed. Systems which employ deterministic as well as random signature sequences are considered. The multiple access capability of noncoherent DS/SS systems is evaluated and compared to that of coherent DS/SS systems with the same parameters. The comparison shows that the loss in the performance of DS/SSMA systems due to noncoherent reception can be considerably larger than the loss incurred to noncoberent single-user systems operating in additive Gaussian noise.

Fiber Optic Configurations for Local Area Networks

Abstract: A number of fiber optic configurations for a new class of demand assignment multiple-access local area networks requiring a physical ordering among stations are proposed. In such networks, the data transmission and linear-ordering functions may be distinguished and be provided by separate data and control subnetworks. The configurations proposed for the data subnetwork are based on the linear, star, and tree topologies. To provide the linear-ordering function, the control subnetwork must always have a linear unidirectional bus structure. Due to the reciprocity and excess loss of optical couplers, the number of stations that can be accommodated on a linear fiber optic bus is severely limited. Two techniques are proposed to overcome this limitation. For each of the data and control subnetwork configurations, the maximum number of stations as a function of the power margin, for both reciprocal and nonreciprocal couplers, is computed.

Coherent Hybrid DS-SFH Spread-Spectrum Multiple-Access Communications

Abstract: The performance of synchronous and asynchronous hybrid direct-sequence/slow-frequency-hopped spread-spectrum multiple-access communications over additive white Gaussian noise channels is examined. Systems employing binary or quaternary phase-shift-keying modulation with coherent demodulation are investigated. Both deterministic and random signature sequences and frequency-hopping patterns are considered and several possible assignments for them are discussed. It is shown that the multiple-access capability of hybrid spread-spectrum is superior to that of pure frequency-hopped spread-spectrum, and inferior to that of pure direct-sequence spread-spectrum for systems with identical bandwidth expansion which employ the same data modulation and demodulation scheme and random hopping patterns and signature sequences.

A Metropolitan Area Network

Abstract: Most high-speed multiple-access networks on the market, or those currently under design, operate in a limited geographical environment and are devoted either to data-only or voice-only transmission. However, challenges issued by the marketplace require the full integration of voice, data, and video in a real-time environment. This paper proposes a metropolitan area network that covers a campus environment and truly integrates voice, data, and video under a single transmission medium. Furthermore, the transmission is achieved through a single media-access schema. This paper also presents marketing issues, network requirements, and technology constraints. The proposed mediaaccess schema is also being considered by the IEEE 802.6 MAN Committee for Standardization.

MAGNET: Columbia's Integrated Network Testbed

Abstract: Fundamental issues arising in the design and implementation of architectures and protocols for integrated local area networks (ILAN's) supporting video, voice, graphics, data, and facsimile, are addressed, and open questions are identified. To characterize the performance of ILAN's, a set of performance evaluation guidelines is established. A class of conceptual solutions for ILAN's is proposed, and its expected performance is obtained by means of simulation. The class includes priority assignment schemes, parametrized sequential and simultaneous scheduling algorithms, and centralized and decentralized access control mechanisms. Our analysis, based upon simulations, shows that the bandwidth freed up by the video and voice calls during periods of little movement in the images and silence periods in the speech signals can be efficiently utilized for graphics and data transmission. The optimal priority mechanism among the different user classes depends on the traffic mix. Finally, it is argued that additional experimentation providing empirical data is imperative for understanding the real-time behavior of ILAN's. Consequently, a network testbed (NTB) called MAGNET is proposed for evaluating the performance of the proposed theoretical systems. It consists of an adaptive ILAN whose mediaaccess protocol is controlled by an Expert System.

An Adaptive Digital Suppression Filter for Direct-Sequence Spread-Spectrum Communications

Abstract: This paper describes the structure of a digital implementation of the Widrow-Hoff LMS algorithm which uses a burst processing technique to obtain some hardware simplification. This adaptive system is used to suppress narrow-band interference in a direct-sequence spread-spectrum communication system. Several different narrow-band interferers are considered, and probability of error results are presented for all cases. While, in general, the results show significant improvement in performance when the LMS algorithm is used, certain disadvantages are also present and are discussed in this paper.

Detection Performance Considerations for Direct-Sequence and Time-Hopping LPI Waveforms

Abstract: The implementation and performance of wide-band detectors for direct-sequence and time-hopping spread-spectrum waveforms in the presence of additive white Gaussian noise are considered in this paper. Of interest here is the performance penalty incurred when going from optimal to suboptimal detector structures. In both cases, performance is quantified by appropriately defined distance measures and is ultimately compared to that of the simplest hypothesis-discriminating device, namely, the energy detector (radiometer).

Voice and Data on a CATV Network

Abstract: A technique for transmitting voice and data on a conventional subsplit CATV network is described. The technique uses a variation on the carrier sense multiple access/collision detection protocol used in local area networks. A variation on this protocol, called movable slot TDM allows periodic voice Sources to share this network without distorting the voice signal. A scheme for sharing frequency spectrum and space is outlined that removes the distance and transmission rate constraints associated with such networks. A transmission strategy is described which allows these protocols to be used on a subsplit CATV system. The system is thus a candidate for local/metropolitan area networks Which support digital voice and data services along with analog (mostly one-way) video.

FH/MFSK Performance in Multitone Jamming

Abstract: The performance of frequency-hopped (FH) M -ary frequency-shift keyed (MFSK) signals in partial-band noise has been extensively analyzed in the open literature. This paper extends the previous research to the usually more effective class of multitone jamming. Specifically, this paper will 1) categorize several different multitone jamming strategies; 2) analyze the performance of FH/MFSK signaling, both uncoded and with diversity, assuming a noncoherent energy-detection metric with linear combining and perfect jamming-state side information, in the presence of worst-case interference for each of these multitone categories; 3) compare the effectiveness of the various multitone jamming techniques, and contrast the results with the partial-band noise jamming case.

A Physical Meteor-Burst Propagation Model and Some Significant Results for Communication System Design

Abstract: This paper describes a meteor-burst communication model based on physical models and astronomical observations of meteor orbits and flux. It compares model results to experimental results and uses them to illuminate this complex propagation medium and to show how "hot spots" of meteor activity vary in position and relative importance with the fundamental meteor-burst link parameters. It also presents antenna and other meteor-burst link design considerations with emphasis on military systems.

The Desktop Computer as a Network Participant

Abstract: A desktop personal computer can be greatly extended in usefulness by attaching it to a local area network and implementing a full set of network protocols, just as one might provide for a mainframe computer. Such protocols are a set of tools that allow the desktop computer not just to access data elsewhere, but to participate in the computing milieu much more intensely. There are two challenges to this proposal. First, a personal computer may often be disconnected from the network, so it cannot track the network state and it must be able to discover and resynchronize with that state very quickly. Second, full protocol implementations have often been large and slow, two attributes that could be fatal in a small computer. This paper reports a network implementation for the IBM Personal Computer that uses several performance-oriented design techniques with wide applicability: an upcall/downcall organization that simplifies structure; implementation layers that do not always coincide with protocol specification layers; copy minimization; and tailoring of protocol implementations with knowledge of the application that will use them. The size and scale of the resulting package of programs, now in use in our laboratory for two years, is quite reasonable for a desktop computer and the techniques developed are applicable to a wider range of network protocol designs.

Methods of Collision Detection in Fiber Optic CSMA/CD Networks

Abstract: Collision detection (CD) is easy to implement in baseband coaxial CSMA/CD networks, but is a formidable design challenge in optical fiber implementations. Seven different methods for detecting collisions in fiber optic networks are described and analyzed. Attention is focused on CD accuracy, CD dynamic range, implementation considerations, and reliability issues.

Tokenless Protocols for Fiber Optic Local Area Networks

Abstract: A family of LAN (Local Area Network) protocols is presented. The LAN consists of a pair of unidirectional fiber optic buses to which stations are connected via passive taps. The protocols provide round-robin bounded delay access to all stations. Contrary to most roundrobin access schemes, the protocols do not require transmission of special packets (tokens); rather, they simply rely on the detection of bus activity at each station. The performance of these protocols in various traffic conditions and system configurations is evaluated via analysis and simulation.

Distributed Switching for Data Transmission over Two-Way CATV

Abstract: Most of tile present approaches to two-way interactive CATV focus on the centralized approach, in which all traffic must travel to the CATV head end. In this paper, we present a distributed approach, in which intelligent store-and-forward packet switches are to be installed at various locations on the cable. The analysis emphasizes three main criteria: the maximum traffic flow on the cable, the throughput of each switch, and the average message delay. Two possible schemes are compared. In one, the switches appear on the main trunk only. In the second, switches are used at branch locations as well.

An Optical Fiber-Based Integrated LAN for MAGNET's Testbed Environment

Abstract: MAGNET is a Network Testbed (NTB) designed to evaluate the performance of local area network architectures and protocols supporting integrated services such as televideo, video conferencing, telephone, computer communication, and facsimile. The integrated local area network (ILAN) presented in this paper has been designed to support the research activities intended for MAGNET. An active slotted ring topology was chosen to implement adaptive sequential-type access mechanisms that are a function of the network loading. The ILAN is based on fiber optic technology with wavelength-division multiplexing of two 100 Mbit/s channels. The media-access protocol provides higher levels with the capability of implementing two modes of operation referred to as circuit-switching, and packet-switching modes, respectively. Provisions have also been made for implementing simultaneous scheduling as well as centralized and decentralized access control schemes. Finally, the reconfigurable structure of the network testbed enables the study of the reliability and availability of ILAN's.

A High Performance LSI Digital Signal Processor for Communication

Abstract: This paper describes a newly developed CMOS LSI DSP (FDSP3). It has a powerful multiplier, which operates at a rate of 10 M-operations/s, almost twice as fast as the fastest existing LSI DSP's. Some key techniques to attain these high performance characteristics of the DSP are described. Suitable memory capacities of the RAM and ROM were analyzed as a function of the operational capability of the DSP. These were reflected in the design of the FDSP3. To assist in the development of the program, support tools have been developed. A Pascal based cross compiler and an on-line debugging tool are described in some detail.

A 200 Mbit/s Synchronous TDM Loop Optical LAN Suitable for Multiservice Integration

Abstract: This paper describes a 200 Mbit/s multiservice optical local area network (LAN) using a synchronous TDM loop structure. The LAN consists of a central supervisory node and multiple service nodes connected by an optical fiber loop. Each service node supports communication channels which have access to allocated time slots in TDM frames continuously circulating on the loop. Multiple independent communication paths of various speeds up to 140 Mbits/s and various modes including point-to-point, ring, and multicast, can be provided between the channels on the loop. The ring will be useful to support ring networks, such as a token ring. The structure of this LAN is quite suitable for integration of multiple services, including video, image, data, and voice, since each service can independently choose its own speed, access method, and mode. In this development, various LSI-based high-speed hardware technologies including compact E/O and O/E modules, GaAs 4 × 4 matrix switch LSI's, and high-speed TDM-processor LSI's, which are versatilely applicable to high-speed LAN's ranging from 100 Mbits/s up to 560 Mbits/s, have been successfully introduced and compact LAN equipment has been obtained. This paper deals mainly with the system and hardware structure of this LAN, together with high-speed hardware technologies. An outline of firmware and network operation, and an application example are also described.

The Analyses of Uncoded Performances for Certain ECCM Receiver Design Strategies for Multihops/Symbol FH/MFSK Waveforms

Abstract: Frequency-hopping (FH) M -ary FSK (MFSK) spread-spectrum modulation is an effective choice for an electronic counter-countermeasures (ECCM) system. When the power efficient MFSK ( M > 2 ) baseband orthogonal waveform is transmitted with L -fold diversity under constrained symbol energy, the resultant multiple ( L ) hops/symbol FH/ MFSK becomes a most viable low probability of intercept (LPI) waveform. In this paper, analyses are presented for the uncoded performance of such a waveform for two types of square-law nonlinear combining soft-decision receiver strategies under the assumption of worst-case partial-band noise jamming (countermeasures). The receivers employ, prior to combining, nonlinear weighting strategies of 1) adaptive gain control and 2) soft limiting (clipping) of the detector output of each channel of the dehopped waveform. The primary focus is on exact analytical results and, as such, the system's thermal noise is not neglected in the analyses. Numerical results of the error rates are graphically displayed as a function of signal-to-jamming power ratio with L and M as parameters under assumed values of signal-to-noise ratio. It is shown that. a diversity gain for error-rate improvement is realized for the nonlinear diversity combining receiver strategies. No attempt is made in this paper to compare the diversity performance to any error-control coding schemes.

A Synchronous Fiber Optic Ring Local Area Network for Multigigabit/s Mixed-Traffic Communication

Abstract: A synchronous-ring fiber optic local area network is reported that facilitates the simultaneous transmission of packet and real-time traffic at gigabit/s rates, minimizes the amount of high-speed logic, and simplifies the user interface to the network. The novelty of the technique is based on 1) suspending in transit around the ring's circumference an integral number of data frames and 2) achieving this condition by skewing the frame clock rate a small amount ( < 1 percent). Rather than use the whole data frame as one packet destined to a specific user (as in a slotted ring), we instead time multiplex many individual channels into the data frame. This technique only becomes feasible for local networks as data rates approach the Gbit/s range. This departure from other synchronous rings results in several advantages both in terms of system performance and hardware simplicity.

Personal Computer Communications Via Ku-Band Small-Earth Stations

Abstract: This paper describes recent developments in geosynchronous satellite communication with special emphasis on data communication using small inexpensive Earth stations (small-Earth stations or microstations). Recent innovations in design, impacts of regulatory codes and decisions, and an illustrative system design example are all examined. Attention is directed to systems using radio frequency (RF) carriers in the Ku -band (14/12 GHz). At some or all of these frequencies terrestrial users are generally considered as secondary to users of fixed satellite services. Because of this relative freedom from terrestrial interference concerns, installation of Ku -band small-Earth stations can be made relatively free of regulatory involvement and coordination procedures. The basic equations governing the noise and interference terms entering link margin calculations are presented. A design example is given, detailing one type of system of practical importance.

Data Link Communications in Tactical Air Command and Control Systems

Abstract: Data links in tactical air command and control systems (ACCS's) connect the computers of command and control units to one another and to weapon systems to enable rapid exchanges of real-time tactical information. This paper, presents an overview description of the current and future ACCS data link architecture used by the Services and NATO. This architecture currently consists of various tactical digital information link (TADIL) standards and NATO link standards. In the near future, the Joint Tactical Information Distribution System (JTIDS) will be a major component of the architecture. While the architecture of the far future is speculative, a description of some concepts being considered, such as the role of the International Standards Organization (ISO) reference model, is presented.

Coding and Modulation Techniques for Frequency-Hopped Spread-Spectrum Communications Over a Pulse-Burst Jammed Rayleigh Fading Channel

Abstract: In this paper, the results of comparing the performance of various forward error correction techniques and several modulation formats when used over a nonselective Rayleigh fading channel in the presence of a pulse-burst jammer are presented. Both binary and nonbinary codes are considered, as well as concatenated codes consisting of either a block or a convolutional inner code and a Reed-Solomon outer code. Finally, the use of side information to allow the decoding of both erasures and errors is also analyzed.

Adaptive Spread-Spectrum Systems Using Least-Squares Lattice Algorithms

Abstract: Practical communication systems must cope with many uncertainties in addition to determining the transmitted data, e.g., the direction, timing, and distortion of the desired signal, and the spectral and spatial distribution of the interference, all of which may change with time. This paper describes exact least-squares (LS) recursive lattice algorithms which resolve these uncertainties in a direct-sequence spread-spectrum digital communication system. The adaptive LS algorithm is recursive beth in order and time, and converges rapidly to the uncertain parameters. Time-discrete algorithms may be mechanized by a receiver containing integrate-and-dump circuits operating at the chip rate of the pseudonoise (PN) sequence, one in each in-phase and each quadrature channel of each sensor array element's output. Different configurations of optimal time-discrete receivers are presented and transformed into adaptive receivers by taking advantage of the spectral properties of the different kinds of LS filters. Simulation results are presented and some guide lines are given for the architecture of an adaptive direct-sequence spread-spectrum system.

Distributed Processing Involving Personal Computers and Mainframe Hosts

Abstract: This paper surveys some of the issues involved in building useful distributed systems involving PC's and hosts. Alternative communications techniques for micro-mainframe communication are compared. The point of view is presented that the PC user should be provided with a unified view of the heterogeneous distributed system to which he is connected. The proposed method is to formalize the notion of a service request and provide distributed services by function shipping service requests to remote nodes able to provide the service, e.g., personal computers will ship requests which they cannot satisfy locally to hosts on the network. Providing a unified view of data which allows PC application programs to access files on mainframes is an example of a service which can be built by intercepting and shipping service requests. Examples from current IBM products are used to illustrate approaches. The views presented are the authors' own, based on systems research in progress at the IBM Thomas J. Watson Research Center.

Propagation Considerations for Satellite Broadcasting at Frequencies Above 10 GHz

Abstract: At frequencies above 10 GHz, tropospheric propagation effects can significantly alter signal transmission performance. Important phenomena on earth-satellite (slant) paths include attenuation and sky noise increases resulting from atmospheric gases, clouds, and precipitation, depolarization by raindrops and ice crystals, and refractive effects, particularly at low path elevation angles. Various practical aspects of propagation phenomena for satellite broadcasting at frequencies above 10 GHz are reviewed in this paper, with emphasis on the near-term implementation Of services at Ku -band.