Showing papers in "IEEE Transactions on Communications in 1988"

Rate-compatible punctured convolutional codes (RCPC codes) and their applications

Abstract: The concept of punctured convolutional codes is extended by punctuating a low-rate 1/N code periodically with period P to obtain a family of codes with rate P/(P+l), where l can be varied between 1 and (N-1)P. A rate-compatibility restriction on the puncturing tables ensures that all code bits of high rate codes are used by the lower-rate codes. This allows transmission of incremental redundancy in ARQ/FEC (automatic repeat request/forward error correction) schemes and continuous rate variation to change from low to high error protection within a data frame. Families of RCPC codes with rates between 8/9 and 1/4 are given for memories M from 3 to 6 (8 to 64 trellis states) together with the relevant distance spectra. These codes are almost as good as the best known general convolutional codes of the respective rates. It is shown that the same Viterbi decoder can be used for all RCPC codes of the same M. the application of RCPC codes to hybrid ARQ/FEC schemes is discussed for Gaussian and Rayleigh fading channels using channel-state information to optimise throughput. >

Image coding using vector quantization: a review

Abstract: A review of vector quantization techniques used for encoding digital images is presented. First, the concept of vector quantization is introduced, then its application to digital images is explained. Spatial, predictive, transform, hybrid, binary, and subband vector quantizers are reviewed. The emphasis is on the usefulness of the vector quantization when it is combined with conventional image coding techniques, or when it is used in different domains. >

Performance models of statistical multiplexing in packet video communications

Abstract: Models and results are presented that assess the performance of statistical multiplexing of independent video sources. Presented results indicate that the probability of buffering (or delaying) video data beyond an acceptable limit drops dramatically as the number of multiplexed sources increases beyond one. This demonstrates that statistical or asynchronous time-division multiplexing (TDM) can efficiently absorb temporal variations of the bit rate of individual sources without the significant variations in reception quality exhibited by multimode videocoders for synchronous TDM or circuit-switched transmission. Two source models are presented. The first model is an autoregressive continuous-state, discrete-time Markov process, which was used to generate source data in simulation experiments. The second model is a discrete-state, continuous-time Markov process that was used in deriving a fluid-flow queuing analysis. The presented study shows that both models generated consistent numerical results in terms of queuing performance. >

Reduced-state sequence estimation with set partitioning and decision feedback

Abstract: A reduced-state sequence estimator for linear intersymbol interference channels is described. The estimator uses a conventional Viterbi algorithm with decision feedback to search a reduced-state subset trellis that is constructed using set-partitioning principles. The complexity of maximum-likelihood sequence estimation (MLSE) due to the length of the channel memory and the size of the signal set is systematically reduced. An error probability analysis shows that a good performance/complexity tradeoff can be obtained. In particular, the results indicate that the required complexity to achieve the performance of MLSE is independent of the size of the signal set for large enough signal sets. Simulation results are provided for two partial-response systems. A simple technique for quadrature partial-response signaling (QPRS) is described that eliminates the quasicatastrophic nature of the ML trellis. >

Signal interception: a unifying theoretical framework for feature detection

Abstract: The unifying framework of the spectral-correlation theory of cyclostationary signals is used to present a broad treatment of weak, random signal detection for interception purposes. The relationships among a variety of previously proposed ad hoc detectors, optimum detectors, and newly proposed detectors are established. The spectral-correlation-plane approach to the interception problem is put forth as especially promising for detection, classification, and estimation in particularly difficult environments involving unknown and changing noise levels and interference activity. A fundamental drawback of the popular radiometric methods in such environments is explained. >

The design of trellis coded MPSK for fading channels: performance criteria

Abstract: It has been well established that the appropriate criterion for optimum trellis-coded modulation design on the additive white Gaussian noise channel is maximization of the free Euclidean distance. It is shown that when the trellis-coded modulation is used on a Rician fading channel with interleaving/deinterleaving, the design of the code of optimum performance is guided by other factors, in particular, the length of the shortest error-event path, and the product of branch distances (possibly normalized by the Euclidean distance of the path) along the path. Although maximum free distance (d/sub free/) is still an important consideration, it plays a less significant role the more severe the fading is on the channel. These considerations lead to the definition of a new distance measure of optimization of trellis codes transmitted over Rician fading channels. If no interleaving/deinterleaving is used, then once again the design of the trellis code is guided by maximizing d/sub free/. >

Digital filter and square timing recovery

Abstract: The digital realization of timing recovery circuits for digital data transmission is considered. A digital algorithm is proposed that can be implemented very efficiently even at high data rates. The resulting timing jitter has been computed and verified by simulations. In contrast to other known algorithms, the one presented here allows free-running sampling oscillators and a novel planar filtering method that prevents synchronization hangups. >

Design of a broadcast packet switching network

Abstract: An overview is given of a system designed to handle a heterogeneous and dynamically changing mix of applications. It is based on fiber-optic transmission systems and high-performance packet switching and can handle applications ranging from low-speed data to voice to full-rate video. A novel feature is a flexible multipoint connection capability suitable for broadcast and conferencing applications. The architecture of a switching systems that can be used to support this network is described. >

Queueing-blocking system with two arrival streams and guard channels

Abstract: An approach to the study of a multichannel cutoff priority system for two Poisson arrival streams with distinct arrival rates and the same potential service time distribution is proposed. This approach makes it possible to obtain the state probabilities in simple closed-form expressions. These expressions provide a straightforward way to derive the distribution of the number of busy servers, the queue length distributions, and, above all, a very simple criterion for the stability of the system. The computations can still be carried out for large systems (e.g. 100 servers). The method can easily be extended from the infinite-queue-length case to a system with finite capacity. As applied to cellular radio, the method offers a means of increasing the total carried traffic while improving the perceived service quality. >

Spreading code protocols for distributed spread-spectrum packet radio networks

Abstract: Spreading code protocols for a distributed spread-spectrum packet radio network are presented. A distributed single-hop system (i.e. each terminal can hear all other terminals) with the users approximately synchronized and a set of prespecified spreading codes are presented. The spreading code protocol is a policy for choosing a spreading code to be used, given that a terminal has a packet to send, and a policy for monitoring spreading codes, given that a terminal is idle. A slotted system where a packet occupies a number of slots is considered, and two protocols that involve changing the spreading code of a transmission after an initial header is transmitted are presented. In one protocol, the header is transmitted on a common code, and in the other it is transmitted on a receiver-based code, the rest of the packet being transmitted on a transmitter-based code. In the receiving mode, a terminal monitors either a common code, in the first case, or a receiver-based code in the latter. Upon recognizing its own address and the source address, the receiver dynamically switches to a despreading code corresponding to the source. Throughput results are obtained for the case of geometrically distributed packet lengths. >

Noncoherent detection of coherent lightwave signals corrupted by phase noise

Abstract: Waves are treated that modulate by either on-off keying (OOK) or binary frequency-shift keying (FSK) and are further impaired by additive Gaussian noise. Heterodyne detection of such a waveform produces an electronic bandpass signal, which, to ease demodulation in the presence of phase noise, is noncoherently demodulated to extract the baseband pulse stream. The treatment goes beyond previous bit error rate (BER) analyses of optical heterodyne receivers for OOK and FSK. First, there is full adherence to the standard (Brownian motion) model of phase noise. Also, the receiver structure is formulated in such a way that the probability density function of the receiver output samples can be accurately determined. This permits calculations of the additive noise and phase noise tolerable when achieving bit error rates as small as 10/sup -9/. Finally, the study is comprehensive regarding the range of parameters explored. Filtering at an intermediate frequency (IF) alone, as well as IF filtering plus postdetection low-pass filtering, is considered. When the receiver parameters decision threshold (for OOK) and IF filter bandwidth are optimized, large amounts of phase noise can be accommodated with only minor increases in required signal-to-noise ratio. This is especially important when the bit rate is moderate compared to the laser linewidth. >

Subband coding of images using vector quantization

Abstract: A novel two-dimensional subband coding technique is presented that can be applied to images as well as speech. A frequency-band decomposition of the image is carried out by means of 2D separable quadrature mirror filters, which split the image spectrum into 16 equal-rate subbands. These 16 parallel subband signals are regarded as a 16-dimensional vector source and coded as such using vector quantization. In the asymptotic case of high bit rates, a theoretical analysis yields that a lower bound to the gain is attainable by choosing this approach over scalar quantization of each subband with an optimal bit allocation. It is shown that vector quantization in this scheme has several advantages over coding the subbands separately. Experimental results are given, and it is shown the scheme has a performance that is comparable to that of more complex coding techniques. >

The design of trellis coded MPSK for fading channels: set partitioning for optimum code design

Abstract: A previous work on criteria for designing trellis-coded MPSK modulation to achieve minimum error probability performance on the Rician fading channel (see ibid., vol.36, no.9, p.1004-1012, Sep. 1988) is extended. It is demonstrated that allowing for multiple symbols per trellis branch, i.e., multiple trellis-coded modulation (MTCM), provides an additional degree of freedom for designing a code to meet the optimization on the fading channel. Diversities larger than those achievable with conventional trellis codes having the same number of trellis states are now attainable, it is under these conditions that MTCM achieves its full potential. >

Accurate method for analysis of a packet-speech multiplexer with limited delay

Abstract: In a packet-speech multiplexer with limited delay, packets arriving once the queue has reached a certain limit are either discarded, or if embedded encoding has been used, shortened. The uniform arrival and service model, which assumes that the information flow in and out of the multiplexer is uniform rather than in discrete packets, is used to analyze such a multiplexer. The equilibrium queue distribution is described by a set of differential equations that can be solved, together with a set of boundary equations describing the queue behavior at its limits, to yield equilibrium distributions of delay and packet loss. Comparisons to simulations using data collected from real conversations show that the packet loss can be determined accurately if the delay limit is less than 400 ms and more than half the packet length. >

Multiple trellis coded modulation (MTCM)

Abstract: The authors demonstrate a trellis coded modulation technique referred to as multiple trellis coded modulation (MTCM) wherein more than one channel symbol per trellis branch is transmitted. They have found simple two-state trellis codes for symmetric MPSK multiple phase-shift keying and AM modulations that can achieve 3-dB gain over uncoded modulation at very high signal-to-noise ratios without bandwidth expansion or reduction in information bit rate. The gain of these codes with respect to previously reported two-state trellis codes is between 1 and 2 dB at very high signal-to-noise ratios, depending on the number of bits per Hertz transmitted. These gains are achieved for those of the equivalent conventional trellis codes with the same number of states in the trellis diagram. The authors note that additional computations per branch are needed for the multiple trellis coding scheme. The concept can be extended to a higher number of states and other types of modulations. >

Single-user detectors for multiuser channels

Abstract: Optimum decentralized demodulation for asynchronous Gaussian multiaccess channels is considered. It is assumed that the receiver is the destination of the information transmitted by only one active user, and single-user detectors that take into account the existence of the other active users in the channel are obtained. The problem considered is one of signal detection in additive colored nonGaussian noise, and attention is focused on one-shot structures where detection of each symbol is based only on the received process during its corresponding interval. Particular emphasis is placed on asymptotically optimum detectors for weak interferers, for CDMA (code-division multiple-access) signature waveforms with long spreading codes, and for low background Gaussian noise level. >

Detection of coded modulation signals on linear, severely distorted channels using decision-feedback noise prediction with interleaving

Abstract: On linear bandlimited Gaussian noise channels with sufficiently high SNR, channel capacity can be approached by combining powerful coded modulation schemes designed for Nyquist channels with the equalization power of decision-feedback equalization (DFE). However, this combination may not be realized in a straightforward manner, since, in general, DFE requires delay-free decisions for feedback, and in a coded system such decisions are not sufficiently reliable. A technique is proposed that combines periodic interleaving with noise-predictive DFE, so that delayed reliable decisions can be used for feedback. When sufficient delay in the interleavers can be tolerated, this technique can attain the DFE performance. On severely distorted channels, modest delays can be sufficient to obtain respectable gains over linear equalization. >

The structure of the limit cycles in sigma delta modulation

Abstract: It is shown that when the input to a sigma-delta modulator is a DC level that can be expressed as a rational number b/a, when normalized with respect to the quantizer step, the output bit string is periodic with a period that is multiple of the denominator a. On the basis of number theory, the structure of these cycles for single-loop modulators is determined and the noise contribution is computed. Around such levels the noise has two peaks, for which the maximum value and the width are proportional to the relative signal bandwidth and to the inverse of the period of the cycle, respectively. The effect of the limit cycles on the performance of the A/D and D/A converters using sigma-delta modulation is discussed. A comparison between single-loop and double-loop modulators from the point of view of this phenomena is made. >

Queueing behavior of statistical multiplexers with correlated inputs

Abstract: The author considers an asynchronous time-division-multiplexing system with a finite number of identical users generating data only during their so-called active periods, which alternate with so-called passive periods. Assuming geometric distributions for the active and passive periods of the users, the author derives a functional equation which describes the full steady-state buffer behavior of the multiplexer. The buffer occupancy at various moments can be derived from this equation and explicit functions of the parameters of the system. The results indicate a strong dependency of the mean buffer occupancy on the actual lengths of the active and passive periods of the users, even for a given mean user activity. >

New phase and frequency detectors for carrier recovery in PSK and QAM systems

Abstract: Phase and frequency detectors (PFDs) are presented that considerably extend the acquisition range of carrier-recovery loops in digital communication systems. Based on a simple modification of conventional phase detectors (PDs), the proposed detectors are applicable to a large variety of modulation schemes, including the popular PSK and QAM signal formats. Their application to QPSK and 16- and 64-QAM is extensively discussed, and simulated frequency-detector (FD) characteristics, as well as acquisition behavior of several PFDs, are reported for QPSK and 16 QAM. The results of an experimental evaluation using a 16-QAM laboratory modem are also reported which show that the detectors increase the acquisition range achievable by conventional PDs by more than one order of magnitude. In PSK, the improved acquisition performance is obtained with no penalty in steady-state phase jitter. In combined amplitude- and phase-shift keying, it generally leads to increased jitter, but this is easily avoided by incorporating a lock indicator and switching back to the original PD after lock is acquired. >

The p/sub i/-persistent protocol for unidirectional broadcast bus networks

Abstract: A protocol for multiaccess communication over unidirectional bus networks is proposed, and its performance capabilities are determined. Under this protocol, time is slotted with a slot equaling a packet's transmission time. A station with a packet to send persists in transmitting its packet in an empty slot with probability p/sub i/ until it is successful. Three criteria for fairness in selection of the p/sub i/ are modeled using Markov chains, which are solved to obtain the proper p/sub i/ that satisfy each fairness criterion. Unlike previous studies of unidirectional bus networks, stations are allowed to buffer more than one packet. The average packet delay for this protocol is bounded, and the maximum achievable throughput approaches unity with increasing buffer size. Further, the protocol provides better delay versus throughput behavior for fixed packet lengths than previous round-robin schemes, its performance is insensitive to bus characteristics, and it appears to be particularly well suited for fiber-optic network applications requiring long distances and high bandwidths. Simulation results that confirm the predicted performance are included. >

Gaussian approximation versus nearly exact performance analysis of optical communication systems with PPM signaling and APD receivers

Abstract: A 25 Mbit/s direct-detection optical communication system that used Q=4 PPM signaling was constructed and its performance measured under laboratory conditions. The system used a single mode AlGaAs laser diode ( lambda =834 nm) and low-noise silicon avalanche photodiode (APD). A procedure is given to numerically compute system performance which uses the nearly exact Webb's approximation of the true Conradi distribution for the APD output that does not require excessive amounts of computer time (a few CPU minutes on VAX 8600 per system operating point). Comparison revealed that modeling the APD output as a Gaussian process under conditions of negligible background radiation and low (less than 10/sup -12/A) APD bulk leakage currents leads to substantial underestimates of optimal APD gain and overestimates of system bit error probability. Examples are given which illustrate the breakdown of the Gaussian approximation in assessing system performance. The measured performance of the system was found to be in excellent agreement with the performance predicted by the nearly exact computational procedure. This system achieved a bit error probability of 10/sup -6/ at a received signal energy corresponding to an average of 60 absorbed photons/bit and optimal APD gain of 700. >

Differential detection of GMSK using decision feedback

Abstract: Differential detector structures that use decision feedback to partially remove the effect of destructive intersymbol interference from minimum-shift keying (MSK) are introduced. In comparison to the conventional two-bit differential detection of GMSK (Gaussian MSK), the proposed structures provide about 4-dB E/sub b//N/sub 0/ improvement when the premodulation filter time-bandwidth product is 0.25. As the premodulation filter becomes narrower, the advantages of the proposed receivers are more pronounced. Thus the bit-error-rate performance of differential detection is brought closer to coherent detection while avoiding the problems associated with the carrier recovery. >

Waiting times in discrete-time cyclic-service systems

Abstract: Single-served, multiqueue systems with cyclic service in discrete time are considered. Nonzero switchover times between consecutive queues are assumed; the service strategies at the various queues may differ. A decomposition for the amount of work in such systems is obtained, leading to an exact expression for a weighted sum of the mean waiting times at the various queues. >

Rapid code acquisition algorithm employing PN matched filters

Abstract: The performance of four algorithms using pseudonoise matched filters (PNMFs), for direct-sequence spread-spectrum systems, is analyzed. They are: parallel search with fix dwell detector (PL-FDD), parallel search with sequential detector (PL-SD), parallel-serial search with fix dwell detector (PS-FDD), and parallel-serial search with sequential detector (PS-SD). The operation characteristic for each detector and the mean acquisition time for each algorithm are derived. All the algorithms are studied in conjunction with the noncoherent integration technique, which enables the system to operate in the presence of data modulation. Several previous proposal using PNMF are seen as special cases of the present algorithms. >

Interpolative vector quantization of color images

Abstract: Interpolative vector quantization has been devised to alleviate the visible block structure of coded images plus the sensitive codebook problems produced by a simple vector quantizer. In addition, the problem of selecting color components for color picture vector quantization is discussed. Computer simulations demonstrate the success of this coding technique for color image compression at approximately 0.3 b/pel. Some background information on vector quantization is provided. >

Analysis of strategies for serial-search spread-spectrum code acquisition-direct approach

Abstract: A 'direct' approach to obtain statistics of the code acquisition time for serial-search spread-spectrum receivers is presented, which combines the idea of algebraic characterization of the search with transform domain methods. This is a very general unified technique, yielding many other known results as special cases. One of the major advantages of the direct approach is that the effect of the search strategy on moments of the acquisition times can be isolated from the effect of the detection/verification logic. This fact will permit one both to analyze all known search strategies-confirming several conclusions regarding their comparative performance-and to propose two novel strategies (alternate search) which outperform the conventional ones when the code rewinding time is small in comparison to the dwell times. Optimum search strategy based on the maximum a posteriori criterion (MAP) is analyzed, thus ensuring an absolute and uniform basis of comparison. The two new strategies perform well by this measure. >

Performance of DS/SSMA communications in impulsive channels. II. Hard-limiting correlation receivers

Abstract: For pt.I see ibid., vol.COM-35, no.11, p.1179-88 (1987). In part I it was demonstrated that impulsive channel noise can be a serious detriment to the performance of direct-sequence spread-spectrum multiple-access (DS/SSMA) communications when conventional linear correlation reception is used. Here, a hard-limiting correlator as an alternative for reception of multiple-access transmission in impulsive channels is considered. For K asynchronous binary PSK DS/SSMA users sharing a linear channel corrupted by impulsive noise that is modeled at the output of the front-end filter of the receiver, techniques are developed for analyzing bit error probabilities of this hard-limiting receiver by exact computation for short spreading sequences, by approximation for longer spreading sequences, and by asymptotic limits for infinitely long spreading sequences. Performance is compared to that of the linear correlator under a variety of conditions, showing that hard-limiting correlation reception can offer substantial improvement over conventional systems in impulsive channels. However, the linear receiver is more effective against multiple-access noise only, and so a tradeoff emerges between rejection of impulsive noise and rejection of multiple-access interference. >

A k shortest path algorithm for adaptive routing in communications networks

Abstract: An efficient and flexible algorithm is presented for finding a k shortest loopless path with distinct initial links from one node to each other node. Low-order polynomial bounds are established for the worst-case time complexity of the algorithm, showing it to offer a substantial improvement over applying known algorithms to the problem. The algorithm can incorporate various extensions, including the ability to handle an algebraic objective, which enhance its applicability to diverse network models. In addition, the k shortest path formulation and algorithm are proposed as a basis for network survivability measures where path length bounds exist. >

The throughput efficiency of the go-back-N ARQ scheme under Markov and related error structures

Abstract: A formula for the go-back-N ARQ (automatic repeat request) scheme applicable to Markov error patterns is derived. It is a generalization of the well-known efficiency formula p/(p+m(1-p)) (where m is the round trip delay in number of block durations and p is the block transmission success probability), and it has been successfully validated against simulation measurements. It is found that for a given error rate, error patterns having zero correlation between successive transmission generally fare better than those with negative correlation, and that error patterns with positive correlation fare better still. It is shown that the present analysis can be extended in a straightforward manner to cope with error patterns of a more complex nature. Simple procedures for numerical evaluation of efficiency under quite general error structures are presented. >