Showing papers in "IEEE Transactions on Communications in 1979"

Image Compression Using Block Truncation Coding

Abstract: A new technique for image compression called Block Truncation Coding (BTC) is presented and compared with transform and other techniques The BTC algorithm uses a two-level (one-bit) nonparametric quantizer that adapts to local properties of the image The quantizer that shows great promise is one which preserves the local sample moments This quantizer produces good quality images that appear to be enhanced at data rates of 15 bits/picture element No large data storage is required, and the computation is small The quantizer is compared with standard (minimum mean-square error and mean absolute error) one-bit quantizers Modifications of the basic BTC algorithm are discussed along with the performance of BTC in the presence of channel errors

Approximate Analysis of General Queuing Networks by Decomposition

Abstract: In this paper an approximate method for the analysis of general queuing networks is proposed. The queuing network is of the open network type, having N single server queuing stations with arbitrary interconnections. Customers may enter the network at any queuing station. The interarrival times of the exogenous arrival processes and the service times at the queuing stations are generally distributed. The analysis is based on the method of decomposition where the total network is broken up into subsystems; e.g., queuing stations of the type GI/G/1 or subnets. The subsystems are analyzed individually by assuming renewal arrival and departure processes. All related processes are considered with respect to their first two moments only. An analysis procedure is reported which reduces the total problem to a number of elementary operations which can be performed efficiently with the aid of a computer. Numerical results are reported together with simulation results to demonstrate the accuracy of the new method. The paper concludes with a short discussion of possible extensions of the method.

A Queueing Network Analysis of Computer Communication Networks with Window Flow Control

Abstract: A computer communication network with window flow control is modeled by a closed multichain queueing network. The severe computational limitation of previous solution algorithms is overcome with a heuristic derived from the recently found mean value analysis. A large numerical example is given.

Reception Through Nakagami Fading Multipath Channels with Random Delays

Abstract: The Nakagami fading distribution is shown to fit empirical results more generally than other distributions. A statistical model for a noisy, Nakagami fading multipath channel is given, following Turin's delay-line model. Optimal receivers are derived for two states of knowledge of the channel-known path delays and random path delays. Upper bounds on the probability of error are computed, for binary, equal-energy, equiprobable signals, which are uniformly orthogonal and have equal, triangular, autocorrelation moduli. Results are graphically displayed and show the dependence of error probability on number of paths, amount of fading and spread of path delays.

An Efficient SS/TDMA Time Slot Assignment Algorithm

Abstract: This paper presents an efficient time slot assignment algorithm for an SS/TDMA system. The technique utilized in the algorithm is a systematic method of finding distinct representatives from the row sets of a traffic matrix. The assignment efficiency resulting from the algorithm is 100% for any traffic matrix. The number of switching modes generated by the algorithm is bounded by n^{2} - 2n + 2 for an n \times n traffic matrix. The computational procedures are illustrated by an example for the Advanced WESTAR system. Also included in the paper are the computer simulation results on the numbers of required switching modes for various simulated traffic matrices.

Generalized TDMA: The Multi-Accessing Tree Protocol

Abstract: The tree algorithm, a new basic protocol for accessing a satellite channel by a large number of independent users, is considered. Previous multi-accessing techniques suffered from long delays, low throughput and/or congestion instabilities. The ALOHA algorithm, for example, when applied to the Poisson source model, has a maximum throughput of .37 packets/slot and is unstable. The tree protocol, under similar conditions, is stable, has a maximum average throughput of .43 and has respectable delay properties. In this protocol, the sources are assigned to the leaves of a tree graph. Message contentions are resolved by systematically moving from node to node through the tree, trying to determine the branches containing the conflicting users. It is shown that the tree protocol is a generalization of TDMA and that an optimum dynamic tree adaptively changes from an essentially random access protocol in light traffic to TDMA in heavy traffic. A major result is that if q . the probability that a user has a packet to transmit, is greater than 1/\sqrt{2} , then TDMA and the optimum tree protocol axe the same. If, on the other hand, q , then the optimum tree protocol is more efficient than TDMA. The maximum average throughput of a system consisting of the optimum tree protocol and a finite number of users is one packet/slot. The tree protocol may be applied either to a direct access system or to a demand assignment access system. Consequently, an example is presented comparing the average delay properties of tree and TDMA protocols when applied to direct access and to reservation access systems.

Simple Approximations of the Error Function Q(x) for Communications Applications

Abstract: Simple analytical upper bounds, approximations and lower bounds on the error function Q(x) are presented and analyzed. A class of functions are given, where the choice of two parameters result in good upper bounds, lower bounds and approximations on Q(x) . The results are given in formulas, parameter tables and graphs. The approximations are suitable for programmable pocket calculators.

A Failsafe Distributed Routing Protocol

Abstract: An algorithm for constructing and adaptively maintaining routing tables in communication networks is presented. The algorithm can be employed in message as well as circuit switching networks, uses distributed computation, provides routing tables that are loop-free for each destination at all times, adapts to changes in network flows, and is completely failsafe. The latter means that after arbitrary failures and additions, the network recovers in finite time in the sense of providing routing paths between all physically connected nodes. For each destination, the routes are independently updated by an update cycle triggered by the destination.

Numerical Evaluation of Correlation Parameters for Optimal Phases of Binary Shift-Register Sequences

Abstract: Correlation parameters that are important for spreadspectrum communications applications are evaluated. The optimal phases (with respect to autocorrelation) are obtained for certain binary shiftregister sequences, including all maximal-length sequences of periods 31, 63, 127, and 255. Selected Gold sequences of period 127 and Kasami sequences of period 255 are also considered. Examples are given to illustrate the use of these data in the selection of signature sequences for spread-spectrum multiple-access communications systems.

Speech Coding

Abstract: We analyze the performance of a one-bit digital matched i t e r (DMF) responding to binary signaling through a noisy multipath channel. The output signal-to-noise ratio (SNR,) plays a key role in the analysis. The S N b performance is fully studied for a two-path channel, both fading and non-fading. For the general N-path channel, results are obtained only for the fading case. All results are conditioned on the knowledge of the time delays of all paths, and are valid only for smaIl input signal-to-noise ratio (SNRi). Most of the results have been verified by computer simulations.

Frequency Detectors for PLL Acquisition in Timing and Carrier Recovery

Abstract: A significant problem in phase-locked loop (PLL) timing and carrier extraction is the initial acquisition. Very narrow loop bandwidths are generally required to control phase jitter, and acquisition may depend on an extremely accurate initial VCO frequency (VCXO) or sweeping. We describe two simply implemented frequency detectors which, when added to the traditional phase detector, can effect acquisition even with very small loop bandwidths and large initial frequency offsets. The first is the quadricorrelator, previously applied to timing recovery by Bellisio, while the second is new, and called a rotational frequency detector. The latter, while limited to lower frequencies and higher signal-to-noise ratios, is suitable for many applications and can be implemented with simpler circuitry.

Channel Equalization Using Adaptive Lattice Algorithms

Abstract: In this paper, a study of adaptive lattice algorithms as applied to channel equalization is presented. The orthogonalization properties of the lattice algorithms make them appear promising for equalizing channels which exhibit heavy amplitude distortion. Furthermore, unlike the majority of other orthogonalization algorithms, the number of operations per update for the adaptive lattice equalizers is linear with respect to the number of equalizer taps.

Combined Source-Channel Coding of Images

Abstract: A combined source-channel coding approach is described for the encoding, transmission and remote reconstruction of image data. The source encoder employs two-dimensional (2-D) differential pulse code modulation (DPCM). This is a relatively efficient encoding scheme in the absence of channel errors. In the presence of channel errors, however, the performance degrades rapidly. By providing error control protection to those encoded bits which contribute most significantly to image reconstruction, it is possible to minimize this degradation without sacrificing transmission bandwidth. The result is a relatively robust design which is reasonably insensitive to channel errors and yet provides performance approaching the rate-distortion bound. Analytical results are provided for assumed 2-D autoregressive image models while simulation results are described for real-world images.

On the Statistical Analysis of Queue Lengths and Waiting Times for Statistical Multiplexers with ARQ Retransmission Schemes

Abstract: The stop-and-wait and continuous error detection retransmission schemes are widely used to control errors in many computercommunication systems. In this paper a model is developed which describes the statistical behavior of the multiplexer using these error detection and retransmission schemes. The model is applied to the case where errors are independent and can occur during data and/or acknowledgment transmission. Numerical results for the mean waiting time and mean queue occupancy are presented for the case of independent errors.

Analysis of Direct-Sequence Spread-Spectrum Multiple-Access Communication Over Rician Fading Channels

Abstract: This paper is concerned with the performance of biphase direct-sequence spread-spectrum multiple-access communication for a general class of fading channels. The channels considered are those for which the channel output consists of a strong stable specular signal plus a faded version of this signal. Such channels, which are referred to as Rician fading (or Rice fading) or specular-plus-Rayleigh fading, are the result of a transmission medium which gives rise to a major stable communication path and a number of additional weaker communication paths. The fading channel is modeled as a general wide-sense-stationary uncorrelatedscattering (WSSUS) channel-a model which is general enough to exhibit both time and frequency selectivity and to impose no restrictions on the fading rate. For the general WSSUS model, results are obtained on the average signal-to-noise ratio at the receiver output in terms of the spread-spectrum signature sequences and the covariance function for the fading process. These results are then specialized to each of two important classes of WSSUS channels: time-selective fading channels and frequency-selective fading channels. Numerical evaluations are presented for specific examples of each of these two types of channels. Analytical expressions are derived for a spread-spectrum multiple-access system with random signature sequences, and the use of these expressions in preliminary system design is discussed.

BRAM: The Broadcast Recognizing Access Method

Abstract: In this paper, we first present the broadcast recognizing access method (BRAM), an access protocol suitable for regulating internode communication in either a radio or (coaxial or fiber) cable based communication system. The method avoids collisions, imposes negligible computational requirements on the nodes attempting to transmit, and is fair in the sense that no node will be indefinitely prevented from transmitting. Next we introduce parametric BRAM which attempts to balance the length of inserted channel idle periods, resulting from scheduling effects, against the probability of allowed message collisions. We show that parametric BRAM can be used to realize a method which balances inserted channel idle time against the probability of message collision to yield enhanced performance. For high message loads, parametric BRAM converges to BRAM, while for low and medium loadings it yields throughputs in excess of BRAM, and other methods. Both BRAM and parametric BRAM are discussed under the assumption of homogeneous message arrival rates at the nodes. We conclude by showing how the parametric BRAM can be applied when the nodes operate with heterogeneous or mixed message arrival rates.

Congestion Control of Store-and-Forward Networks by Input Buffer Limits--An Analysis

Abstract: The use of input buffer limits for congestion control of store-and-forward networks is investigated. An analytic model is formulated. Based upon the analytic results, strategies are proposed for the design of input buffer limits to achieve the maximum network throughput as well as to provide a safety margin for uncertainties in traffic assumptions. A useful capacity law is discovered. Major conclusions drawn from the analysis are supported by simulation results for a fournode homogeneous network. These results indicate that input buffer limits which satisfy the capacity law are a simple and effective means of network congestion control. Further simulation studies are underway to investigate methods of implementation in a general network.

A Comparison of Modulation Techniques for Digital Radio

Abstract: This paper describes and summarizes the characteristics of the modulation techniques most applicable to digital radio. The modulation techniques discussed are on-off-keying (OOK) with coherent and noncoherent detection, quadrature amplitude modulation (QAM), quadrature partial response (QPR), frequency-shift-keying (FSK) with noncoherent detection, continuous phase FSK (CP-FSK) with coherent and noncoherent detection, minimum-shift-keying (MSK), binary and quaternary phase-shift-keying (BPSK, QPSK) with coherent and differentially coherent detection, offset-keyed QPSK (OK-QPSK), M -ary PSK with coherent detection ( M = 8, 16 ), and 16-ary amplitude and phase-shift-keying (APK). Functional descriptions of these schemes are provided and their performance is compared in a series of tables summarizing the results of the literature of the past 20 years. The modulation schemes are compared with respect to ideal (white Gaussian noise) performance, spectral properties, signaling speed, complexity, and the effects on performance of interference, fading and delay distortion.

The Single Server Queue with Periodic Arrival Process and Deterministic Service Times

Abstract: We derive an algorithm for computing the exact delay distribution for the following first-come-first-served single server queue. The arrival process is the superposition of N + 1 independent equivalent deterministic arrival streams, and the service times are deterministic. Numerical results are used to compare this system with an approximating system which has been used in the absence of exact results: the M/D/1 queue (Poisson input, deterministic service time). A comparison with a finite source model is also given. This study was motivated by the following issue germane to packet switching data networks. There are N + 1 packet processes, arriving over identical trunks, which must share the transmission capacity of a single high-speed trunk in a store-and-forward manner. The idealized queueing model above analyzes the worst-delay situation for this problem when the incoming trunks are fully utilized and the packets are maximum sized. The results indicate that an M/D/1 approximation can be quite pessimistic in predicting the performance of such a system.

The Stutter Go Back-N ARQ Protocol

Abstract: In this paper we consider the problem of designing a good ARQ protocol for a message transmission environment characterized by high error rates and/or long propagation delays. We derive the average queue length for an idealized ARQ protocol for such an environment. We also describe a modification that can be made to existing ARQ protocols which can significantly decrease queue lengths in such an environment. We show that for the case of low message traffic rates, the modified Go Back- N protocol approaches the idealized scheme in performance.

Performance and Optimization of Switched Diversity Systems for the Detection of Signals with Rayleigh Fading

Abstract: The performance and optimization of switched diversity systems are considered. First, the one-dimensional distribution and probability density functions of the envelope of the received signal are obtained for three different switching strategies. This information is used to obtain the average probability of bit error for the case of non-coherent detection of binary FSK signals with Rayleigh fading envelopes and additive white Gaussian noise. The optimization of two of these switching strategies is then considered, and it is shown that by proper selection of switching thresholds, the average probability of bit error during detection can be minimized. It is also shown that these optimized switching strategies yield a significant improvement in performance over non-diversity systems and can approach the performance of more complex receivers such as maximal ratio combining. Computer simulations of switched diversity systems using a practical field model are used to verify the analysis.

Specification and Validation of Protocols

Abstract: Complex protocols are used to coordinate remote activities in computer networks. To insure proper operation, formal techniques of protocol definition and validation have been proposed, and developed to the point that they can be applied to actual protocols. However, much work remains to be done in order to cope with protocols of ever-increasing complexity; in particular, those coordinating the activities of many interacting entities. The characteristics that determine the applicability of the different specification and validation techniques to a protocol will be discussed. We will define the "topology" of a protocol, and treat also protocols that are intended to work in a variety of configurations, as well as configurations which may change in time (i.e. "evolving topologies"). Finally, based on this new general point of view, a short survey of specification and validation techniques will be presented, and the extensions needed to handle complex protocol characteristics will be discussed.

Resynch Procedures and a Fail-Safe Network Protocol

Abstract: In this paper a new class of network synchronization procedures, called Resynch Procedures, is described. A resynch procedure is a mechanism for effectively bringing all nodes of a distributed network to a known state simultaneously, despite arbitrary finite delays between nodes. The procedures presented have the interesting property that no time-outs are required. One use of a resynch procedure is to implement a network protocol that can guarantee that no packets will be lost and no duplicate packets will be inadvertently received, despite arbitrary node and link failures. This appears to be the first demonstration that such fail-safe protocols exist.

Improvements in Block-Retransmission Schemes

Abstract: Consider the case where an n -digit block encoded word cannot be decoded reliably and a second block of n redundant digits is sent to allow the receiver to make a new try based on the combined information received. Two classes of schemes are proposed and analyzed which give significantly better performance than is obtained by sending a repeat of the first block, yet do not require excessive decoding complexity. One approach is to consider small sub-blocks of the original n -digit code as the data digits of a short rate one-half code. The other approach is to treat the first sending as the data digits of a systematic convolutional code of short constraint length. Comparisons are made for the white Gaussian noise channel and the erasure channel. The comparisons are limited to an assessment of the improvement gained after two sendings. Procedures using length 4 sub-blocks and using convolutional codes with constraint lengths as short as 2 or 3 digits yield considerable improvement over block retransmission. For the case of the erasure channel, a very simple decoding rule is devised for the convolutional code case.

Message Delays in FDMA and TDMA Communication Channels

Abstract: Message delays under a Time Division Multiple Access (TDMA) scheme and the corresponding Frequency Division Multiple Access (FDMA) scheme are studied and compared. Under a TDMA scheme, a network station is assigned a number of slots for each time frame. Under the corresponding FDMA scheme, this station is allocated a separate frequency band for which the ratio between its width and the channel bandwidth is equal to the corresponding time portion allocated by the TDMA scheme. The distribution of the message delay difference under the corresponding TDMA and FDMA schemes, for any message arrival stream, any service ordering discipline and at any time, is derived. This distribution is shown to be equal to that of a simple random variable associated with the message arrival stream. Message delays under a TDMA scheme are shown to be always lower than those under the corresponding FDMA scheme, but the difference value is lower than the time frame duration. Station-buffer queue sizes under both schemes are shown to be essentially the same.

Differential Detection of MSK with Nonredundant Error Correction

Abstract: A proposed scheme for differential detection of MSK with nonredundant error correction utilizes the output detected from the difference in phase between alternate signaling intervals, along with the output of the conventional differential detector, in accordance with the concept of a convolutional error correcting code. Theoretical analysis and experimental results show that the degradation from coherent detection is about 0.1 dB at a 10 dB signal-to-noise ratio.

Polar Quantization of a Complex Gaussian Random Variable

Abstract: We solve numerically the optimum fixed-level non-uniform and uniform quantization of a circularly symmetric complex (or bivariate) Gaussian random variable for the mean absolute squared error criterion. For a given number of total levels, we determine its factorization into the product of numbers of magnitude and phase levels that produces the minimum distortion. We tabulate the results for numbers of "useful" output levels up to 1024, giving their optimal factorizations, minimum distortion, and entropy. For uncoded quantizer outputs, we find that the optimal splitting of rate between magnitude and phase, averaging to 1.52 and 1.47 bits more in the phase angle than magnitude for optimum and uniform quantization, respectively, compares well with the optimal polar coding formula Of 1.376 bits of Pearlman and Gray [3]. We also compare the performance of polar to rectangular quantization by real and imaginary parts for both uncoded and coded output levels. We find that, for coded outputs, both polar quantizers are outperformed by the rectangular ones, whose distortion-rate curves nearly coincide with Pearlman and Gray's polar coding bound. For uncoded outputs, however, we determine that the polar quantizers surpass in performance their rectangular counterparts for all useful rates above 6.0 bits for both optimum and uniform quantization. Below this rate, the respective polar quantizers are either slightly inferior or comparable.

Optimal Blocklengths for ARQ Error Control Schemes

Abstract: Optimal blocklengths are investigated for various ARQ schemes used for error control in data transmission systems in independent-bit-error and Poisson-distributed block-error channels. A number of results relating to optimum block length for various ARQ schemes are collected for easy reference. New results are obtained for some of these schemes. The product of this investigation is suggested procedures for computing optimal blocklengths. Examples are given.

Performance of Optical Receivers with Avalanche Photodetection

Abstract: This paper reports the results of a computer study to numerically evaluate the digital performance of an optical receiver with avalanche photodetectors. Exact and approximate APD statistics were implemented and error probabilities in the range l0^{-1} to l0^{-4} were computed. Both shot-noise and receiver-noise limited conditions were examined, and on-off keying, binary pulse comparison, multilevel intensity, and PPM signaling formats are considered.

Satellite Packet Communication--Multiple Access Protocols and Performance

Abstract: Satellite communication systems have traditionally been designed for voice traffic. Multiple access protocols for conflict resolution have typically been channel-oriented with either fixed or demand assignment. Data communications, however, have much more diverse traffic characteristics and transmission requirements than voice communications. We present in this paper an overview of two major categories of packet-oriented multiple access protocols: contention and reservation protocols. A traffic model suitable for the data communications environment is first introduced. A key element in this model is user-specified message delay constraints. Our primary performance measure of a protocol is the channel throughput versus average message delay tradeoff characteristic. The main attributes of the two categories of packet-oriented protocols are discussed. Four specific contention protocols are described and their performance characteristics are examined. Design considerations of the two important components of reservation protocols, reservation channel and distributed global queue, are discussed. Three reservation protocols with distributed control are described. Finally the performance of channel-oriented protocols and the two classes of packet-oriented protocols are compared using a variety of traffic models.