Showing papers in "IEEE Transactions on Communications in 1987"
TL;DR: Two simple models of queueing on an N \times N space-division packet switch are examined, and it is possible to slightly increase utilization of the output trunks and drop interfering packets at the end of each time slot, rather than storing them in the input queues.
Abstract: Two simple models of queueing on an N \times N space-division packet switch are examined. The switch operates synchronously with fixed-length packets; during each time slot, packets may arrive on any inputs addressed to any outputs. Because packet arrivals to the switch are unscheduled, more than one packet may arrive for the same output during the same time slot, making queueing unavoidable. Mean queue lengths are always greater for queueing on inputs than for queueing on outputs, and the output queues saturate only as the utilization approaches unity. Input queues, on the other hand, saturate at a utilization that depends on N , but is approximately (2 -\sqrt{2}) = 0.586 when N is large. If output trunk utilization is the primary consideration, it is possible to slightly increase utilization of the output trunks-upto (1 - e^{-1}) = 0.632 as N \rightarrow \infty -by dropping interfering packets at the end of each time slot, rather than storing them in the input queues. This improvement is possible, however, only when the utilization of the input trunks exceeds a second critical threshold-approximately ln (1 +\sqrt{2}) = 0.881 for large N .
1,592 citations
TL;DR: In this article, the authors present the results of indoor radio propagation measurements of two signal distribution approaches that improve the picture dramatically and show that, with either approach, the signal attenuation can be reduced by as much as a few tens of decibels and the rms delay spread becomes limited to 20 to 50 us, even in large buildings.
Abstract: The idea of implementing an indoor radio communications system serving an entire building from a single central antenna appears to be an attractive proposition. However, based on various indoor propagation measurements of the signal attenuation and the multipath delay spread, such a centralized approach appears to be limited to small buildings and to narrow-band FDMA-type systems with limited reliability and flexibility. In this paper, we present the results of indoor radio propagation measurements of two signal distribution approaches that improve the picture dramatically. In the first, the building is divided into many small cells, each served from an antenna located in its own center, and with adjacent cells operating in different frequency bands. In the second approach, the building is divided into one or more large cells, each served from a distributed antenna system or a "leaky feeder" that winds its way through the hallways. This approach eliminates the frequency cell handoff problem that is bound to exist in the first approach, while still preserving the dramatic reductions in multipath delay spread and signal attenuation compared to a centralized system. For example, the measurements show that, with either approach, the signal attenuation can be reduced by as much as a few tens of decibels and the rms delay spread becomes limited to 20 to 50 us, even in large buildings. This can make possible the implementation of sophisticated broad-band TDMA-type systems that are flexible, robust, and virtually building-independent.
568 citations
TL;DR: It is shown that the standard decision-directed estimatedgradient adaptation algorithm for joint MSE equalization and carrier recovery can be turned into an algorithm providing effective blind convergence in the MSE sense, usable in the closed-eye startup phase with no need of a known training sequence.
Abstract: We show that the standard decision-directed estimatedgradient adaptation algorithm for joint MSE equalization and carrier recovery, normally utilized in the open-eye condition, can be turned into an algorithm providing effective blind convergence in the MSE sense, usable in the closed-eye startup phase with no need of a known training sequence. This is obtained by means of a simple flag telling both the equalizer and the synchronizer whether the current output error with respect to the decided symbol is sufficiently reliable to be used. If not, adaptation is stopped for the current iteration. In the paper, this "stopand-go" decision-directed algorithm is presented for both linear and decision-feedback MSE complex equalizers with joint blind carrier recovery. Simulation results demonstrate the effectiveness of the proposed technique.
460 citations
TL;DR: By attacking the problem of congestion for the infinite-storage case, this work discovers new solutions applicable to switches with finite storage.
Abstract: Most prior work on congestion in datagram systems focuses on buffer management. We find it illuminating to consider the case of a packet switch with infinite storage. Such a packet switch can never run out of buffers. It can, however, still become congested. The meaning of congestion in an infinite-storage system is explored. We demonstrate the unexpected result that a datagram network with infinite storage, first-in, first-out queueing, at least two packet switches, and a finite packet lifetime will, under overload, drop all packets. By attacking the problem of congestion for the infinite-storage case, we discover new solutions applicable to switches with finite storage.
423 citations
TL;DR: As a continuation of Part I, the spectral correlation function is presented for a variety of types of digitally modulated signals, including digital pulse-amplitude, pulse-width, and pulse-position modulation, and various types of phase- shift keying and frequency-shift keying.
Abstract: As a continuation of Part I, the spectral correlation function is presented for a variety of types of digitally modulated signals. These include digital pulse-amplitude, pulse-width, and pulse-position modulation, and various types of phase-shift keying and frequency-shift keying. The magnitudes of the spectral correlation functions are graphed as the heights of surfaces above a bifrequency plane, and these graphs are used as visual aids for comparison and contrast of the spectral correlation properties of different modulation types.
413 citations
TL;DR: An expression for the output of the receiver is obtained for the case of random signature sequences, and the corresponding characteristic function is determined to study the density function of the multiple-access interference and to determine arbitrarily tight upper and lower bounds on the average probability of error.
Abstract: Binary direct-sequence spread-spectrum multiple-access communications, an additive white Gaussian noise channel, and a coherent correlation receiver are considered. An expression for the output of the receiver is obtained for the case of random signature sequences, and the corresponding characteristic function is determined. The expression is used to study the density function of the multiple-access interference and to determine arbitrarily tight upper and lower bounds on the average probability of error. The bounds, which are obtained without making a Gaussian approximation, are compared to results obtained using a Gaussian approximation. The effects of transmitter power, the length of the signature sequences, and the number of interfering transmitters are illustrated. Each transmitter is assumed to have the same power, although the general approach can accommodate the case of transmitters with unequal powers.
402 citations
TL;DR: Several distributed routing rules are investigated that take advantage of the regular structure of the Manhattan Street Network, and two procedures are described that allow the network to adapt to node or link failures.
Abstract: The Manhattan Street Network is a regular, two-connected network, designed for packet communications in a local or metropolitan area. It operates as a slotted system, similar to conventional loop networks. Unlike loop networks, routing decisions must be made at every node in this network. In this paper, several distributed routing rules are investigated that take advantage of the regular structure of the network. In an operational network, irregularities occur in the structure because of the addressing mechanisms, adding single nodes, and failures. A fractional addressing scheme is described that makes it possible to add new rows or columns to the network without changing the addresses of existing nodes. A technique is described for adding one node at a time to the network, while changing only two existing links. Finally, two procedures are described that allow the network to adapt to node or link failures. The effect that irregularities have on routing mechanisms designed for a regular structure is investigated.
378 citations
TL;DR: This paper rigorously derive several basic properties of a simple discrete-time single integrator loop sigma-delta modulator with an accumulate-and-dump demodulator and shows that when the input is constant, the state sequence of the integrator in the encoder loop can be modeled exactly as a linear system in an appropriate space.
Abstract: Oversampled sigma-delta modulation has been proposed as a practical implementation for high rate analog-to-digital conversion because of its simplicity and its robustness against circuit imperfections. To date, mathematical developments of the basic properties of such systems have been based either on simplified continuous-time approximate models or on linearized discrete-time models where the quantizer is replaced by an additive white uniform noise source. In this paper, we rigorously derive several basic properties of a simple discrete-time single integrator loop sigma-delta modulator with an accumulate-and-dump demodulator. The derivation does not require any assumptions on the correlation or distribution of the quantizer error, and hence involves no linearization of the nonlinear system, but it does show that when the input is constant, the state sequence of the integrator in the encoder loop can be modeled exactly as a linear system in an appropriate space. Two basic properties are developed: 1) the behavior of the sigma-delta quantizer when driven by a constant input and its relation to uniform quantization, and 2) the rate-distortion tradeoffs between the oversampling ratio and the average mean-squared quantization error.
373 citations
TL;DR: The central role of spectral correlation, in the characterization of random processes that are cyclostationary in the wide sense, is explained and a spectral correlation function that is a generalization of the power spectral density function is described.
Abstract: The importance of the concept of cyclostationarity in design and analysis of signal detectors, synchronizers, and extractors in communication systems is briefly discussed, and the central role of spectral correlation, in the characterization of random processes that are cyclostationary in the wide sense, is explained. A spectral correlation function that is a generalization of the power spectral density function is described, and a corresponding generalization of the Wiener-Khinchine relation and several other fundamental spectral correlation relations also are described. Explicit formulas for the spectral correlation function for various types of analog-modulated signals are derived. This includes pulse and carrier amplitude modulation, quadrature amplitude carrier modulation, and phase and frequency carrier modulation. To illustrate the differing spectral correlation characteristics of different modulation types, the magnitudes of the spectral correlation functions are graphed or described in graphical terms as the heights of surfaces above a bifrequency plane.
307 citations
TL;DR: In this paper, a Markov model is used to develop a product form solution to analyze the throughput of arbitrary topology multihop packet radio networks that employ a carrier sensing multiple access (CSMA) protocol with perfect capture.
Abstract: In this paper, we use a Markov model to develop a product form solution to efficiently analyze the throughput of arbitrary topology multihop packet radio networks that employ a carrier sensing multiple access (CSMA) protocol with perfect capture. We consider both exponential and nonexponential packet length distributions. Our method preserves the dependence between nodes, characteristic of CSMA, and determines the joint probability that nodes are transmitting. The product form analysis provides the basis for an automated algorithm that determines the maximum throughput in networks of size up to 100 radio nodes. Numerical examples for several networks are presented. This model has led to many theoretical and practical extensions. These include determination of conditions for product form analysis to hold, extension to other access protocols, and consideration of acknowledgments.
293 citations
TL;DR: In this paper, the performance of a single link between a user and its receiver in the central station, and consider two types of diversity, selection diversity and predetection combining to exploit the multipath channel, is investigated.
Abstract: Direct-sequence spread spectrum with differential phase shift-keying (DPSK) modulation and code-division multiple-access is a promising approach for wireless communications in an indoor environment, which is characterized in this paper by a Rayleigh-fading multipath channel. In this study, we consider two specific channel models having different path-delay distributions and average path power profiles. A star configuration, in which each user exercises average power control in transmitting to a central station, is the basic communication unit, which could be one cell in a cellular hierarchy. We obtain the performance of a single link between a user and its receiver in the central station, and consider two types of diversity, selection diversity and predetection combining to exploit the multipath. A similar system with coherent PSK (CPSK) modulation has been studied previously for one of the channel models considered here. For the same channel model, we show that the irreducible error probability with selection diversity is about half an order of magnitude higher when DPSK is used instead of CPSK. With predetection combining, the performance improves significantly in comparison with selection diversity as the diversity order increases. DPSK modulation with predetection combining is akin to coherent PSK with optimal maximal-ratio combining, but is simpler to implement. The performance with selection diversity for a second channel model, which is based on measurements in an office building, is not significantly different. This indicates that the spreadspectrum approach is rather robust to the path-delay distribution and average path-power profile.
TL;DR: These novel architectures for rearrangeably nonblocking multistage photonic space switches implemented using arrays of Ti:LiNbO_{3} directional couplers have a controllable amount of crosstalk, low optical path loss, and an asymptotically optimal number of directional coupler switches for a given switch size.
Abstract: We present some novel architectures for rearrangeably nonblocking multistage photonic space switches implemented using arrays of Ti:LiNbO_{3} directional couplers. Multistage networks, studied mostly in the electronic domain, are obtained by minimizing the number of 2 × 2 elements needed to implement a switch. Unfortunately, straightforward extensions of these networks to the photonic domain show that the switch size has to be severely limited by the crosstalk in each of the Ti:LiNbO_{3} 2 \times 2 switching elements. Our networks, on the other hand, have a controllable (including almost zero) amount of crosstalk, low optical path loss, and an asymptotically optimal number of directional coupler switches for a given switch size. In addition, the switch has a simple control algorithm and its performance for light loading appears very promising. The switch is easily decomposable into smaller arrays of no more than two types, making it easy to partition the switch into chips. At the cost of a slight increase in crosstalk, the switch can be made single fault tolerant in terms of its ability to connect any input to any output.
TL;DR: Results on the packet error probability for frequency-hop and direct-sequence spread spectrum systems are presented and it is shown that the assumption of independent errors out of the decoder also gives an upper bound.
Abstract: The use of convolutional coding in packet radio systems introduces problems in evaluating performance, since the errors out of the decoder are not independent. A new bound on the packet error probability out of a Viterbi decoder may be used to evaluate these systems. We present results on the packet error probability for frequencyhop and direct-sequence spread-spectrum systems. In addition, we show that the assumption of independent errors out of the decoder gives an upper bound that is not as tight as our new bound. Comparisons are made with frequency-hop systems using Reed-Solomon codes.
TL;DR: This paper proves that FDDI timing requirements are satisfied, i.e., the token rotates quickly enough to prevent initiation of recovery unless there is failure of a physical resource or unless the network management entity within a station initiates the recovery process.
Abstract: The fiber distributed data interface (FDDI) is an ANSI draft proposed standard for a 100 Mbit/s fiber-optic token ring. The FDDI timed token access protocol provides dynamic adjustment of the load offered to the ring, with the goal of maintaining a specified token rotation time and of providing a guaranteed upper bound on time between successive arrivals of the token at a station. FDDI also provides automatic recovery when errors occur. The bound on time between successive token arrivals is guaranteed only if the token rotates quickly enough to satisfy timer requirements in each station when all ring resources are functioning properly. Otherwise, recovery would be initiated unnecessarily. The purpose of this paper is to prove that FDDI timing requirements are satisfied, i.e., the token rotates quickly enough to prevent initiation of recovery unless there is failure of a physical resource or unless the network management entity within a station initiates the recovery process.
TL;DR: With optimum combining, radio communications can be used in high-density, multiple-user environments, such as within buildings, even when only limited bandwidth is available.
Abstract: This paper studies the use of optimum combining to increase the capacity of narrow-band in-building radio communication systems with multiple users. We consider systems consisting of a base Station with numerous remotes in a Rayleigh fading environment and study the problem of more users requiring channels than the number of channels available. A system is described that, with multiple antennas at the base station but only one antenna at each remote, uses optimum combining to suppress interfering signals. We show that this system, with M antennas at the base station, can achieve an M -fold increase in the number of users or tolerate M - 1 interferers from other systems. Thus, with optimum combining, radio communications can be used in high-density, multiple-user environments, such as within buildings, even when only limited bandwidth is available.
TL;DR: This paper formulate the problem of deciding on the sequence of page transmissions as a Markovian decision process and shows that, from a response time point-of-view, a cyclic order ofpage transmissions is optimal.
Abstract: Teletext is a one-way information delivery system where pages of information are broadcast to all users in a continuous manner. System response time is an important consideration in the design of teletext systems. One factor contributing to response time is the order in which pages are transmitted. In this paper, we formulate the problem of determining the sequence of page transmissions as a Markovian decision process. Using this formulation we show that, from a response time point of view, a cyclic order of page transmissions is optimal. We also describe two algorithms for designing a teletext broadcast cycle.
TL;DR: These results show that in each case the high signal-to-noise maximum-likelihood rules have a performance nearly equal to that of the maximum- likelihood rules over a wide range of practically interesting signal- to- noise ratios (SNR's).
Abstract: The problem of locating a periodically inserted frame synchronization pattern in random data for a M -ary digital communication system operating over the additive white Gaussian noise channel is considered. The optimum maximum-likelihood decision rule, high signal-to-noise approximate maximum likelihood decision rule, and ordinary correlation decision rule for frame synchronization are derived for both coherent and noncoherent phase demodulation. A general lower bound on synchronization probability is derived for the coherent correlation rule. Monte Carlo computer simulations of all three decision rules, along with evaluations of the lower bound for the coherent correlation rule, were performed for the coherent MPSK, coherent, and noncoherent M ary orthogonal, and 16 QAM signaling schemes. These results show that in each case the high signal-to-noise maximum-likelihood rules have a performance nearly equal to that of the maximum-likelihood rules over a wide range of practically interesting signal-to-noise ratios (SNR's). These high SNR decision rules also provide significant performance improvement over the simple correlation rules. Moreover, they are much simpler to implement than the maximum-likelihood decision rules and, in fact, are no more complex than the correlation rules.
TL;DR: This paper analyzes a direct-sequence, spread-spectrum, multiple-access SSMA communication system which assigns a set of M orthogonal sequences to each user, and obtains approximations for the multiuser probability of error by using a Gaussian approximation for the multiple- access interference.
Abstract: This paper analyzes a direct-sequence, spread-spectrum, multiple-access (SSMA) communication system which assigns a set of M orthogonal sequences to each user. With all direct sequence SSMA systems, K users share a channel by phase modulating their transmissions with signature sequences. However, the users of our system transmit log_{2}M bits of information/sequence. This contrasts classical SSMA schemes which use a pair of antipodal sequences and transmit 1 bit/sequence. In this paper, we assume that the channel noise is a combination of additive white Gaussian noise (AWGN) and multiple-access interference. We employ the optimum (single-user) demodulator for orthogonal signals in Gaussian noise. The multiple-user performance of this receiver is analyzed. We obtain approximations for the multiuser probability of error by using a Gaussian approximation for the multiple-access interference. We also obtain an upper bound on the exact probability by using characteristic functions. Our SSMA system is Well suited for application at the lower radio frequencies. Therefore, a companion paper describes a realistic model for low-frequency radio noise, modifies the receiver to include a zero-memory nonlinearity, and studies the performance of the nonlinear receiver.
TL;DR: A distributed algorithm for obtaining a fair time slot allocation for link activation in a multihop radio network and introduces the concept of maximal fairness in which the termination of a fair allocation algorithm is related to maximal reuse of the channel under a given fairness metric.
Abstract: We present a distributed algorithm for obtaining a fair time slot allocation for link activation in a multihop radio network. We introduce the concept of maximal fairness in which the termination of a fair allocation algorithm is related to maximal reuse of the channel under a given fairness metric. The fairness metric can be freely interpreted as the expected link traffic load demands, link priorities, etc. Since respective demands for time slot allocation will not necessarily be equal, we define fairness in terms of the closeness of allocation to respective link demands while preserving the collision free property. The algorithm can be used in conjunction with existing link activation algorithms to provide a fairer and fuller utilization of the channel.
TL;DR: In this article, a method for determining the error probability of a receiver using differential detection in the presence of Gaussian noise and fast Rician fading is presented, which includes the effect of IF filter distortion.
Abstract: A method is presented for determining the error probability of a receiver using differential detection in the presence of Gaussian noise and fast Rician fading. Equations for the covariances of the fading component are derived, which include the effect of IF filter distortion. It is shown that these equations may be readily evaluated numerically. A simple formula for the error probability is derived for systems using BPSK and a matched filter receiver. An example of the error probability is given using this receiver. Also given is an example of a system using MSK with a practical IF filter. Different spectral shapes and bandwidths for the fading process are investigated for this example and their effect on the error probability is determined.
TL;DR: This paper considers the joint design of n/(n + 1) trellis codes and asymmetric 2^{n+1} -point signal constellations, which has no bandwidth expansion relative to an uncoded 2n-point symmetric signal set, and considers the tradeoff between the additional savings in required E_{b}/N_{0} and the additional complexity needed to achieve it.
Abstract: Traditionally symmetric signal constellations, i.e., those with uniformly spaced signal points, have been used for both uncoded and coded systems. Although symmetric signal constellations are optimum with no coding, the same is not necessarily true for coded systems. This paper shows that by designing the signal constellations to be asymmetric, one can, in many instances, obtain a performance gain over the traditional symmetric constellations combined With trellis coding. In particular, we consider the joint design of n/(n + 1) trellis codes and asymmetric 2^{n+1} -point signal constellations, which has no bandwidth expansion relative to an uncoded 2n-point symmetric signal set. The asymptotic performance gains due to coding and asymmetry are evaluated in terms of the minimum free Euclidean distance d free of the trellis. A comparison of the maximum value of this performance measure to the minimum distance d min , of the uncoded system is an indication of the maxiamm reduction in required E_{b}/N_{0} that can be achieved for arbitrarily small system bit error rates. Bit error probability analysis is carried out for general cases. A few examples are given to show the performance gain due to the asymmetry of the signal set. It is to be emphasized that the introduction of asymmetry into the signal set does not affect the bandwidth or power requirements of the system; hence, the abovementioned improvements in performance come at little or no cost. Asymmetric signal sets in coded systems first appear in the work of Divsalar and Yuen [1], [2]. Here we expand upon these results by considering various types of asymmetric signal sets combined with the optimum (in the sense of maximum d free ) trellis code having 2, 4, 8, and 16 states. The numerical results obtained will clearly demonstrate the tradeoff between the additional savings in required E_{b}/N_{0} and the additional complexity (more trellis states) needed to achieve it.
TL;DR: In this article, the performance of digital linear correlation receivers is studied in a multiuser environment, where there are assumed to be two types of sources interfering with data transmission: multiple access interference and additive channel noise which is attributed to impulsive noise sources in the environment.
Abstract: The performance of digital linear correlation receivers is studied in a multiuser environment. There are assumed to be two types of sources interfering with data transmission: multiple-access interference, and additive channel noise which is attributed to impulsive noise sources in the environment. The contribution of multiple-access interference is examined by considering K asynchronous users transmitting simultaneously over a linear channel using the binary PSK direct-sequence spreadspectrum multiple-access (DS/SSMA) technique. Alternatively, the effects of the non-Gaussian impulsive channel in such a system are studied by modeling the samples of noise after front-end filtering. Errorprobability performance under these conditions is compared to that for additive white Gaussian noise (AWGN) channels. Due to computational complexity, exact analysis is limited here to systems utilizing short spreading sequences. Computationally simple methods are proposed for approximating the average error probability when the length of the signature sequences is large. Furthermore, some asymptotic results are obtained for the case of infinitely long sequences. In all cases, performance variation is examined as the shape of the noise density varies with SNR held constant. The results of this analysis indicate that the presence of impulsive noise can cause significant performance degradation over that predicted from an AWGN model, even when the total noise power does not increase.
TL;DR: It is shown that partial symmetry in the polling table and the station characteristics can be used to significantly reduce the number of equations which must be solved in a polling system such as a token bus or token ring with exhaustive service and priority polling.
Abstract: This paper derives exact results for a polling system such as a token bus or token ring with exhaustive service and priority polling. The results can also be used to analyze a terminal controller with a generalservice order table. There are N stations in the system and the token is passed among them according to a polling table of length M (\geq N) . Stations are given higher priority by being listed more frequently in the polling table. By a straightforward extension of results of Ferguson and Aminetzah [5] for systems with circular polling and exhaustive service, it is shown that in general, the N mean waiting times require the solution of a set of M - N simultaneous equations and a set of M(M - 1) simultaneous equations. We show that partial symmetry in the polling table and the station characteristics can be used to significantly reduce the number of equations which must be solved. We present the reduced equation set for a two-priority class system and apply the results to a large token-passing bus network in which a few nodes account for a substantial portion of the network traffic. We show that in the latter case, the overall_ average message waitmg time can be significantly reduced by using priority polling: average waiting times at the high-priority nodes have large reductions in return for a smaller increase at low-priority nodes.
TL;DR: In this paper, the authors describe source encoding of the outputs of a block truncation coder, namely, the overhead statistical information and the truncated block, which can be effectively used for their quantization as vectors.
Abstract: This paper describes source encoding of the outputs of a block truncation coder (BTC), namely, the overhead statistical information and the truncated block. The statistical overhead and the truncated block exhibit properties which can be effectively used for their quantization as vectors. Vector quantization of these BTC outputs results into reduction of the bit rate of the coder. The bit rate reduces up to 1.5 bits/ pel if vector quantization is used on one of the outputs; i.e., either the overhead information or the truncated block. By vector quantizing both the BTC outputs the bit rate can he reduced up to 1.0 bits/pel without introducing many perceivable errors in the reconstructed output.
TL;DR: It is found that the variance improvement may be severely limited by the dimensionality of the system, and a means for circumventing this limitation is described through the definition of a statistically equivalent impulse response.
Abstract: The assessment of bit error rate (BER) performance of a digital communication system via computer simulation has traditionally been done using the Monte Carlo method. For very low BER, this method requires excessive computer time. This time can be substantially reduced by using extrapolation based on importance sampling (IS). In applying IS to a complex system, many considerations must be addressed, chief among which is the reliability (variance) of the estimator as a function of the system particulars. We discuss a number of these considerations and, specifically, derive a number of expressions for the variance. We find that the variance improvement may be severely limited by the dimensionality (or memory) of the system. We describe a means for circumventing this limitation through the definition of a statistically equivalent impulse response. For a linear system, this amounts to the ordinary impulse response. The simulation can be structured to estimate the equivalent impulse response using statistical regression. This new approach has been implemented and found to yield significant runtime improvement over conventional importance sampling for linear systems of large dimensionality. We believe this technique will work also for mildly nonlinear systems, as might be encountered in typical satellite Communications.
IBM1
TL;DR: Algorithms for rapid gain acquisition and the detection of a cyclic training signal in the presence of noise and carrier-frequency offset are presented, and it is shown that the equalizer coefficients can be computed efficiently by spectral division.
Abstract: Fast initial training of a voiceband data-modem receiver which employs an equalizer with fractionl- T spaced coefficients is presented. The modem receiver performs rapid gain acquisition, and monitors the equalizer delay line for the presence of a periodic training signal whose period is equal to the length of the equalizer delay line. Testing for periodicity is combined with estimating carrier-frequency offset. Once a full period of the training signal has been obtained, the equalizer coefficients are computed by spectral division, and coefficient centering is performed. The receiver then proceeds in a reference-directed mode until the end of the training signal is detected. In the paper, algorithms for rapid gain acquisition and the detection of a cyclic training signal in the presence of noise and carrier-frequency offset are presented. Cyclic training of an equalizer with fractional- T spaced coefficients is derived. It is shown that the equalizer coefficients can be computed efficiently by spectral division. Coefficient centering, carrier-phase and symbol synchronization, reference-directed training, and the transition to data mode are described. Finally, measurement results for an experimental 2400 baud modem using an equalizer with 64 T/2 -spaced coefficients are given. The modem achieves a startup time of 20 ms on a Bell 3002 unconditioned leased line.
TL;DR: In this article, the analysis of a multipath-combining receiver for direct-sequence spread-spectrum communications through a specular multipath channel is developed, and the performance of the system is determined in terms of parameters of the signature sequences.
Abstract: The analysis of a multipath-combining receiver for directsequence spread-spectrum communications through a specular multipath channel is developed. The analysis applies to systems that use quadriphase-shift-keyed, offset quadriphase-shift-keyed, minimum-shiftkeyed, or binary phase-shift-keyed modulation. The measures of performance are the signal-to-noise ratio and approximations to the error probability involving the signal-to-noise ratio. The performance of a multipath-combining receiver is determined not only for the case of a single transmitter, but also for the case of multiple interfering transmit, ters. Furthermore, the performance of the system is determined in terms of parameters of the signature sequences. These parameters can be used as guides in selecting signature sequences for the system. Results are also given for the case of randomly generated signature sequences.
TL;DR: Refinement of the CFA optimization techhique to allow optimal discretization of the continuous solutions to discrete solutions is presented based on dynamic programming and iterations of Flow Optimization and Capacity Optimization.
Abstract: This paper presents a global optimal solution technique for the joint optimization of link capacity and flow assignment (CFA) in a packet-switched communications network, m-M/M/1 queueing systems to model existing networks are considered. A continuous lower bound of the average packet delay is used in the formulation of the cost objective function for the capacity and flow assignment. The cost objective function thus formulated is shown to be convex with respect to the network multicommodity flow. This convexity ensures the global optimal solution to the CFA problem via the flow deviation (FD) method. Refinement of the CFA optimization techhique to allow optimal discretization of the continuous solutions to discrete solutions is presented based on dynamic programming and iterations of Flow Optimization and Capacity Optimization. Application examples of these optimization procedures are illustrated.
IBM1
TL;DR: Simple mathematical models and simulation indicate a potential throughput advantage over slotted ALOHA and CSMA in a quasi-orthogonal packet radio network.
Abstract: Quasi-orthogonal codes are assigned to transmitters in a packet radio network such that interference caused by hidden terminals is eliminated. In addition, a handshaking protocol permits random access between nodes. Simple mathematical models and simulation indicate a potential throughput advantage over slotted ALOHA and CSMA.
TL;DR: In this article, the authors developed a mobile satellite channel simulator in hardware to enable system designers and technologists to optimize their design and pursue their research and development, the simulator has sufficient flexibility to facilitate many forms of tests and experiments.
Abstract: In designing a mobile satellite network, engineers and technologists are faced with wide-ranging issues for which there is no prior database. System engineers must address such issues as adequate margin to combat multipath fading, the level of adjacent channel protection required to allow transmission in narrow-band channels, the cochannel protection required to allow for frequency reuse in a multiplebeam system, and the level of intermodulation distortion tolerable for single-channel-per-carrier operation. Technologists, on the other hand, must determine the performance of various system components. For example, in the ground segment, modem and speech codec performance must be evaluated in the presence of thermal noise, fading, and other impairments. To enable system designers and technologists to optimize their design and pursue their research and development, the Jet Propulsion Laboratory has developed a mobile satellite channel simulator in hardware. This simulator has sufficient flexibility to facilitate many forms of tests and experiments.