Showing papers in "IEEE Transactions on Communications in 1978"
TL;DR: An N -point discrete Fourier transform (DFT) algorithm can be used to evaluate a discrete cosine transform by a simple rearrangement of the input data.
Abstract: An N -point discrete Fourier transform (DFT) algorithm can be used to evaluate a discrete cosine transform by a simple rearrangement of the input data. This method is about two times faster compared to the conventional method which uses a 2N -point DFT.
327 citations
TL;DR: This paper considers the problem of channel assignment in mobile communication systems, where the service area is divided in hexagonal cells, and a Hybrid Channel Assignment Scheme is studied.
Abstract: This paper considers the problem of channel assignment in mobile communication systems, where the service area is divided in hexagonal cells. In particular, a Hybrid Channel Assignment Scheme is studied and certain results are obtained using GPSS simulation of a model 40-cell system.
307 citations
TL;DR: This work shows how certain "fast recursive estimation" techniques, originally introduced by Morf and Ljung, can be adapted to the equalizer adjustment problem, resulting in the same fast convergence as the conventional Kalman implementation, but with far fewer operations per iteration.
Abstract: Very rapid initial convergence of the equalizer tap coefficients is a requirement of many data communication systems which employ adaptive equalizers to minimize intersymbol interference. As shown in recent papers by Godard, and by Gitlin and Magee, a recursive least squares estimation algorithm, which is a special case of the Kalman estimation algorithm, is applicable to the estimation of the optimal (minimum MSE) set of tap coefficients. It was furthermore shown to yield much faster equalizer convergence than that achieved by the simple estimated gradient algorithm, especially for severely distorted channels. We show how certain "fast recursive estimation" techniques, originally introduced by Morf and Ljung, can be adapted to the equalizer adjustment problem, resulting in the same fast convergence as the conventional Kalman implementation, but with far fewer operations per iteration (proportional to the number of equalizer taps, rather than the square of the number of equalizer taps). These fast algorithms, applicable to both linear and decision feedback equalizers, exploit a certain shift-invariance property of successive equalizer contents. The rapid convergence properties of the "fast Kalman" adaptation algorithm are confirmed by simulation.
307 citations
TL;DR: This paper describes a new type of frequency modulation, called Tamed Frequency Modulation (TFM), for digital transmission, where the desired constraint of a constant envelope signal is combined with a maximum of spectrum economy which is of great importance, particularly in radio channels.
Abstract: This paper describes a new type of frequency modulation, called Tamed Frequency Modulation (TFM), for digital transmission. The desired constraint of a constant envelope signal is combined with a maximum of spectrum economy which is of great importance, particularly in radio channels. The out-of-band radiation is substantially less as compared with other known constant envelope modulation techniques. With synchronous detection, a penalty of only 1 dB in error performance is encountered as compared with four-phase modulation. The idea behind TFM is the proper control of the frequency of the transmitter oscillator, such that the phase of the modulated signal becomes a smooth function of time with correlative properties. Simple and flexible implementation schemes are described.
247 citations
TL;DR: In optical communication, ideal amplification of the received signal leads to a limiting signaling rate of 1 nat per photon, which is much inferior to the optimum limit of kT joules/nat, which the authors can theoretically approach by counting photons.
Abstract: In optical communication, ideal amplification of the received signal leads to a limiting signaling rate of 1 nat per photon. This is much inferior to the optimum limit of kT joules/nat, which we can theoretically approach by counting photons. Practically, the rates we can attain by photon counting will be limited by how elaborate codes we can instrument rather than by thermal photons.
245 citations
TL;DR: A recently constructed 12-channel digital echo canceler that interfaces directly with the 8-bit μ255 PCM now standard for digital transmission in the telephone plant is described.
Abstract: We describe a recently constructed 12-channel digital echo canceler that interfaces directly with the 8-bit μ255 PCM now standard for digital transmission in the telephone plant. The four most interesting features of the canceler are time sharing of circuitry to reduce per channel costs, floating-point multiplication, loop-gain normalization, and the use of a test channel for fault detection. Extensive laboratory and field testing has shown the canceler to be working well.
234 citations
TL;DR: A technique for reducing overhead is presented together with the results of analysis and simulation and it is shown that with reduced overhead there is a reduction in the average delay of messages.
Abstract: A critical component of computer communications networks is local distribution, i.e., techniques for connecting geographically dispersed users to a central facility. A drawback to conventional polling techniques for local distribution is excessive overhead. Thus, in systems with many lightly loaded terminals, message delay is more a function of the time required to poll all terminals than of traffic from competing sources. A technique for reducing overhead is presented together with the results of analysis and simulation. The technique identifies terminals having messages by a process of elimination starting with a poll of groups of terminals. Further, the technique is adaptive in that the sizes of groups to be polled are chosen according to the probability of a terminal having a message. The object of the adaptivity is to minimize the average time required to examine all terminals. The results of analysis and simulation show considerable reduction in this average for systems with many lightly loaded terminals. Moreover, the adaptive feature insures that there is no penalty for heavy loading. With reduced overhead there is a reduction in the average delay of messages.
215 citations
TL;DR: A dynamic routing policy where messages that arrive at a certain node are routed to leave the node on the link having the shorter queue, and it is found that the average delay for the dynamic system is better by a factor of K.
Abstract: Diffusion theory has sometimes been successful in providing excellent approximate solutions to difficult queueing problems. Here we explore whether such methods can be used to analyze a basic dynamic routing strategy associated with a single idealized node in a data network. We analyze a dynamic routing policy where messages, or packets, that arrive at a certain node are routed to leave the node on the link having the shorter queue. In the model, message or packet arrivals are Poisson and the service time is exponentially distributed. We explore a heavy traffic diffusion method and we also discuss the limitations of an ad hoc approach to applying diffusion. For a node with K outgoing queues we find, under the assumption of heavy traffic, the optimum dynamic strategy, in the sense of minimizing the average delay. When this optimum dynamic strategy is compared to a static strategy where the outgoing traffic is split among the K queues, we find that the average delay for the dynamic system is better by a factor of K .
198 citations
TL;DR: This paper presents differential encoding techniques which can be used with a variety of symmetric signal sets to remove their phase ambiguity, and these techniques do have low performance penalties relative to the uncoded performance.
Abstract: Because of the symmetry in most two-dimensional signal constellations, ambiguities exist at the receiver as to the exact phase orientation of the received signal set. In PSK systems, this ambiguity is resolved by the use of differential encoding. This paper presents differential encoding techniques which can be used with a variety of symmetric signal sets to remove their phase ambiguity. While not proven to be optimum, the techniques do have low performance penalties relative to the uncoded performance. The key to reducing the performance penalty is to use the minimum amount of differential encoding necessary to resolve the ambiguity. Examples of encoding techniques for several common signal constellations are given, including their performance penalties.
177 citations
IBM1
TL;DR: Under conditions likely to be encountered on actual voiceband communication channels, the clock phase derived is shown to prevent spectral nulls and to accurately approximate the optimum timing phase for an infinite equalizer.
Abstract: The performance of conventional modem receivers, where adaptive equalization is achieved by a digital transversal filter with tap gains spaced at the symbol interval, depends critically on the choice of the sampling phase. In this paper, a digital timing recovery loop is described and analyzed in the case of passband quadrature amplitude modulated data signals. Under conditions likely to be encountered on actual voiceband communication channels, the clock phase derived is shown to prevent spectral nulls and to accurately approximate the optimum timing phase for an infinite equalizer. Computer simulations show that the proposed system is capable of fast timing acquisition.
163 citations
TL;DR: The system performance of the present Fibernet experiment, which uses a 19-port transmissive star coupler, GaAIAs injection lasers and avalanche photodiodes, and incorporates bi-phase data encoding is described, and the merits and problems of linear, ring and several star configurations are compared.
Abstract: Local computer networks which communicate over copper conductors have been developed both to promote resource sharing and provide increased performance. Such networks typically operate at bandwidth-length ( Bw \circ L ) products up to a few MHz \circ km . In this paper we consider the use of fiber optics in such networks, and give a status report on a star-configured fiber optic network experiment called Fibernet which operates at a Bw \circ L product of \sim 100 MHz\circ km at a data rate of 150 Mbits/s and which in its final phases will connect up to 19 stations. We compare the merits and problems of linear, ring and several star configurations, and of active versus passive networks. The packet communication protocol is discussed and network efficiency is calculated as a function of the packet length, channel capacity and network propagation time. We describe the system performance of the present Fibernet experiment, which uses a 19-port transmissive star coupler, GaAIAs injection lasers and avalanche photodiodes, and incorporates bi-phase data encoding. Power distribution inhomogeneities, observed in the output of the transmissive star coupler's mixer rod, are explained geometric-optically.
TL;DR: In a single packet switch with a finite number of packet buffers shared between several output queues, it is shown that restricted sharing prevents congestion by making throughput an increasing function of load.
Abstract: Consider a single packet switch with a finite number of packet buffers shared between several output queues. An arriving packet is lost if no free buffer is available, as in the CIGALE network. It has been observed by simulation that if load increases too much, congestion may occur, i.e., throughput declines; it appears that the busiest link's queue tends to hog the buffers. Therefore, we will limit the queue length and when the queue is full the packet will be dropped. We expect that this restricted buffer sharing policy will avoid congestion under conditions of heavy load. A queueing model of a packet switch is defined and solved by local balance. Loss probability is evaluated, and values of queue limit to minimize loss are found; they depend on load. A Square-Root rule is introduced to make the choice of queue limit independent of load. For a sample switch, with three output links, a comparison is made between performance under different buffer sharing policies; it is shown that restricted sharing prevents congestion by making throughput an increasing function of load.
TL;DR: Narrowband jamming and interference due to other users in spread spectrum communication systems can be effectively suppressed by using digital whitening, and an impressive improvement in receiver performance can be obtained.
Abstract: Narrowband jamming and interference due to other users in spread spectrum communication systems can be effectively suppressed by using digital whitening. As a result, an impressive improvement in receiver performance can be obtained. The digital whitening is accomplished using a transversal filter whose coefficients are selected by either a Wiener algorithm or a maximum entropy algorithm. Filters obtained by use of these algorithms are evaluated for various jamming and signaling conditions and are found to exhibit comparable performance over a wide range of input signal-to-noise ratios.
TL;DR: This paper presents a comprehensive review of the ARPANET routing algorithm, from its original implementation to the authors' plans for future modifications, and describes in detail subsequent modifications and the actual implementation currently in use.
Abstract: This paper presents a comprehensive review of the ARPANET routing algorithm, from its original implementation to our plans for future modifications. We hope that by collecting this information, and by providing considerable details, we can provide others with a useful reference document concerning some of the practical problems of network algorithm design. Much of the discussion below assumes a basic familiarity with the principles of packet switching, the ARPANET implementation, and some of the relevant terminology, information which can be found, for example, in [4]. Sections 1 and 2 give a brief summary of basic routing concepts and of the original routing algorithm, respectively. The following two sections describe in detail subsequent modifications and the actual implementation currently in use. Section 5 then discusses some problems that have developed over the past few years, as network usage has grown considerably. The final sections outline some explanations for these problems and some mechanisms for improving performance. We are in the process of implementing these and other changes to the routing algorithm.
TL;DR: A simple Markov birth-death model is presented for the random process a(t) representing the number of talkers issuing talkspurts at a given time, and it is shown to hold independently of the probability density function of talkspurt duration.
Abstract: The cutout fraction in a TASI system is shown to be \phi=\frac{1}{np}\sum_{k=c+1}^n (k-c) n\choose k p^{k}(1-p)^{n-k} where n is the number of sources, c is the number of channels, and p is the probability that a source is issuing a talkspurt at a random time. This result is shown to hold independently of the probability density function of talkspurt duration. The same formula is shown to apply to the fraction of packets lost in a packet-switched link with a transmission capacity of c packets every T p seconds, where T p is the interval between packet generations for an individual source during talkspurt, and where no packet is queued for a time longer than T p . In addition, a simple Markov birth-death model is presented for the random process a(t) representing the number of talkers issuing talkspurts at a given time.
TL;DR: Constraints on capacity allocation are investigated for circuit-switched demand access to a common transmission resource by user communities with differing traffic intensity and capacity requirements.
Abstract: Constraints on capacity allocation are investigated for circuit-switched demand access to a common transmission resource by user communities with differing traffic intensity and capacity requirements. A simple birth-death steady-state traffic model is used together with a geometrical representation of any given set of constraints placed upon user access. State probabilities have a simple product form which holds for a wide class of constraint sets. Performance characteristics are intrinsic to the upper surface of the constraint set.
TL;DR: A modification to the basic Go-Back- N ARQ error control technique is described which yields improved throughput efficiency performance for all block error rates.
Abstract: A modification to the basic Go-Back- N ARQ error control technique is described which yields improved throughput efficiency performance for all block error rates.
TL;DR: Through this study, it has become clear that WDM technologies play a major role in optical fiber systems and have the possibilities of realizing the various optical fiber transmission systems.
Abstract: This paper describes the feasibility and the applicability of the Wavelength-Division-Multiplexing (WDM) system with two types of preliminary WDM transmission experiments. Through this study, it has become clear that WDM technologies play a major role in optical fiber systems and have the possibilities of realizing the various optical fiber transmission systems.
TL;DR: A method of recovering bandlimited signals from unequally spaced samples is described and repeated application of this process is shown to converge to the original signal.
Abstract: A method of recovering bandlimited signals from unequally spaced samples is described. The method uses an iterative procedure which requires low pass filtering of the unequally spaced sample pulses. This is followed by resampling at the same unequally spaced times and again low pass filtering to obtain a correction signal. Repeated application of this process is shown to converge to the original signal. These results are obtained by slight modifications of the analysis in Reference 2.
TL;DR: This paper looks at three techniques for choosing a DIR, choosing the DIR by truncation, minimum mean square error and matching the power spectrum to that of the original channel.
Abstract: Equalizer structures using the Viterbi Algorithm achieve at least order of magnitude performance improvement over linear equalizers on some intersymbol interference channels. Using a linear equalizer to shape the original channel impulse response to some shorter desired impulse response (DIR) is a technique which reduces the complexity of the Viterbi Algorithm equalizer. This paper looks at three techniques for choosing a DIR. These are choosing the DIR by truncation, minimum mean square error and matching the power spectrum to that of the original channel. Using effective signal to noise ratio as the figure of merit for comparison, results are given for one particular channel.
TL;DR: Techniques applied to the digital processing of speech and signaling in a 60 channel FDM to 30 channel PCM converter are presented and adequacy of the various options retained for the digitalprocessing is confirmed by the experimental results.
Abstract: Techniques applied to the digital processing of speech and signaling in a 60 channel FDM to 30 channel PCM converter are presented. Great efficiency is achieved through connecting a particular type of Fourier Transform computer to a polyphase network in which coefficient symmetries are exploited. Digital processing parameter values have been chosen so as to meet specified performance objectives. Realization of a laboratory model is outlined and measured performance is reported. Adequacy of the various options retained for the digital processing is confirmed by the experimental results.
TL;DR: A single-moment method for determining the node-to-node grades of service of a circuit-switched communication network employing a routing strategy called "originating-office control with spill-forward" is presented.
Abstract: This paper presents a single-moment method for determining the node-to-node grades of service of a circuit-switched communication network employing a routing strategy called "originating-office control with spill-forward." An example of such a network is the European AUTOVON. The new method makes use of the concept of "path-loss sequence" and the techniques in system reliability analysis to determine the probability of each route being used to complete a call. Results from a digital computer implementation of the method and comparisons with other methods are discussed.
TL;DR: The effect on channel capacity of the overhead created by the error-control traffic for both slotted ALOHA and carrier sense multiple access (CSMA) is studied.
Abstract: We consider a population of terminals communicating with each other or with a central station over a packet-switched multiple access radio channel. To ensure the integrity of the transmitted data over the multi-access channel, we consider a reliable method using an error detecting block code in conjunction with a positive acknowledgment of each correct message. In this paper, we study the effect on channel capacity of the overhead created by the error-control traffic for both slotted ALOHA [1] and carrier sense multiple access (CSMA) [2]. For this we consider several implementation schemes for the two channel configurations: the common-channel configuration (a single channel for both information traffic and error-control traffic); and the split-channel configuration. The packet delay analysis will be treated in a forthcoming companion paper.
TL;DR: This paper investigates the optimum entropy versus distortion performance of quantizers optimized for uniform, Gaussian, Laplacian, and gamma-distributed memoryless sources which are useful models of the quantizer input signals in speech or picture coding schemes.
Abstract: This paper investigates the optimum entropy versus distortion performance of quantizers optimized for uniform, Gaussian, Laplacian, and gamma-distributed memoryless sources which are useful models of the quantizer input signals in speech or picture coding schemes. We list the maximally obtainable signal-to-quantization noise ratios for one-dimensional optimum (i.e. entropy-coded) quantizers in the important low bit-rate region. These results have been obtained by an iterative solution of a set of nonlinear equations. Additionally we have also computed the corresponding rate-distortion functions by employing the Blahut-algorithm. These latter results upperbound the performances of multi-dimensional quantization schemes, and a comparison with the former results indicates the penalty to be paid for restricting a coder to perform a one-dimensionai quantization. It will be shown that the differences can be significant in the low bit-rate region.
TL;DR: The analysis shows that the mutual interference problem is less severe with users employing synchronous FH than with the other spread spectrum techniques.
Abstract: This paper considers the mutual interference problem of several users employing the same spread spectrum technique in selected multiple user environments. The spread spectrum techniques consist of pseudo noise (PN), time division multiple access/PN, synchronous and asynchronous frequency hopping (FH). The environment consists of a desired transmitter-receiver pair located in an area where there are M interfering users distributed in accordance with a specified probability density function. Both coherent phase-shift-keyed-and noncoherent frequency-shift-keyed modulations are considered. The general relationship between the probability of bit error of PN and FH systems is derived which is independent of the signal modulation and distribution of users. The degradation of the communication system performance (average probability of bit error) of the desired link as a function of the total number of interfering users within the considered area is investigated. The analysis shows that the mutual interference problem is less severe with users employing synchronous FH than with the other spread spectrum techniques. The comparison between asynchronous FH and PN is highly dependent on the relative location of interferers to the desired link and the time duty factor of the hopping.
TL;DR: The average error probabilities and outage rates of error probability for the M -phase CPSK signal through the Nakagami's m -distributed fading channel are exactly evaluated both for nondiversity reception and for diversity reception.
Abstract: The average error probabilities and outage rates of error probability for the M -phase CPSK signal through the Nakagami's m -distributed fading channel are exactly evaluated both for nondiversity reception and for diversity reception. The probability density functions of the composite phase of fading signal and noise and those of the diversity-combined signal envelopes are newly derived in this paper. The results are generally obtained including the digital phase modulation component, the fading figure as a measure of fading depth, the average carrier-to-noise power ratio, the power correlation coefficient between two diversity branches, etc. The diversity improvements are also verified. Additionally some useful approximate formulas are briefly shown. This study will add a newly widened view to the considerations of system performances and designs for digital radio communications via fading channels.
TL;DR: The capabilities and design of a new digital video system which permits a user to record and playback sequences of digitized color television signal and is intended as a simulation tool for investigating the effects of various video coding and processing algorithms.
Abstract: This paper describes the capabilities and design of a new digital video system which permits a user to record and playback sequences, up to 80 s long, of digitized color television signal. In addition, a user may process the stored video via the associated computer and view the results. The above system, which is called the DVS (Digital Video Store), is intended as a simulation tool for investigating the effects of various video coding and processing algorithms.
TL;DR: A unified treatment is provided of the methods for computing terminal reliability based on recursive case analysis, and the possible choices in case enumeration are discussed, and a rationale is given to support a particular policy.
Abstract: In this paper a unified treatment is provided of the methods for computing terminal reliability based on recursive case analysis. The possible choices in case enumeration are discussed, and a rationale is given to support a particular policy. Simplification and decomposition techniques are also examined, and some experimental results are described, obtained with a computer program, which is a good compromise between efficiency and simplicity.
IBM1
TL;DR: An implementation of a recent theory of Zafiropulo is described, which defines protocols in terms of the interaction between two directed graphs, and uses set theory and predicate logic to determine all error conditions that can arise.
Abstract: An interaction between two communicating processes can be defined in terms of a protocol or set of rules governing the exchange of messages between them. For any given protocol, it is a significant problem to determine whether or not errors can occur when the processes interact. In this paper, we describe an implementation of a recent theory of Zafiropulo, which defines protocols in terms of the interaction between two directed graphs, and uses set theory and predicate logic to determine all error conditions that can arise. The overall structure of the theory is used, but the determination of the error conditions is based on a graphical representation of the interaction, and particular emphasis is placed on the state of the channel between the two processes. The technique is currently limited to the validation of two-process protocols in which the processes return to an initial state after a finite number Of interaction steps. The implementation demonstrates that a completely automated procedure can be defined which finds a significant class of errors in communications protocols.
IBM1
TL;DR: A technique is described which identifies design errors in protocols based on modeling a protocol as a pair of interacting graphs whereby interaction sequences are represented by path pairs (one path in each graph).
Abstract: A protocol is a set of rules which governs the interaction between processes. It is difficult to design protocols without errors because there are usually more interactions possible than anticipated. We are concerned with two-process protocols. A technique is described which identifies design errors in protocols. The technique is based on modeling a protocol as a pair of interacting graphs whereby interaction sequences are represented by path pairs (one path in each graph). The technique is currently limited to protocols that must revert to an initial or quiescent state after a finite number of interaction steps. The work represents a theory that can be automated on a digital computer.