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It is known that the "better than" raised-cosine pulse has several advantages over the conventional raised-cosine pulse. In addition to the advantages previously reported, this letter reports that the "better than" raised-cosine pulse enables better performance of symbol timing estimation than the raisedcosine pulse. Both Cramer-Rao lower bounds and simulations of practical synchronizers show the superiority of the new pulse. The performance gain can be as much as 3.4 dB for a practical system. Index Terms--"Better than" raised-cosine pulse, Cramer-Rao lower bound, non-data-aided feedforward symbol timing estimation, raised-cosine pulse.

By combining the technique of observer design in nonlinear control systems theory and the technique of phase space reconstruction in nonlinear dynamical systems theory, we show that synchronization of smooth (hyper)chaotic systems can be attained with any scalar transmitted signal. The proposed method has been illustrated via two examples. Index Terms - Chaos, hyperchaos, synchronization theory.

As indicated in ITU-T SG XVIII Rec. G.826 (July 1993 version) an important study is the relationship between the Synchronous Digital Hierarchy (SDH) path level performance monitoring and the block based parameters. In this paper the measure of errored blocks using one second aggregated counts of bit interleaved parity (BIP) failures, as proposed in the Annex 3 of Rec. G.826, is reported and analyzed with Poisson error distribution and error burst distribution.

A second-order DPLL with time-varying loop gains is applied to the symbol synchronization of burst mode data signals. An algorithm to control the DPLL loop gains is derived from adaptive Kalman filtering theory. Simulation results for the variable gain DPLL compared to a fixed gain DPLL demonstrate the improved acquisition performance.

The generation of transmission schedules for self-organizing radio networks by traffic sensitive algorithms is described. A centralized 'Traffic' algorithm that can be used as a performance benchmark is presented. Also described is a distributed 'degree' algorithm that is a traffic sensitized version of an algorithm developed by Ephremides and Truong. Two performance measures for comparing schedules and simulation results are also presented.

In this paper, we propose a new frequency synchronization algorithm for orthogonal frequency division multiplexing (OFDM) systems requiring only one training symbol, based on a conventional method which requires two training symbols. While the timing synchronization is obtained by using the conventional method, the carrier frequency offset is efficiently estimated by the proposed method. It is shown that the proposed method not only reduces the number of the training symbols but also possesses better performance than the conventional method without increase in complexity. Index Terms--Frequency synchronization, orthogonal frequency division multiplexing (OFDM).

A new design method is presented for the design of phase locked-loop filters for carrier recovery, bit timing, or other synchronization loops given phase noise spectrum and noise level. Unlike the conventional designs, our design incorporates a possible large decision delay and S-curve slope uncertainty. Large decision delays frequently exists in modern receivers. Due to, for example, a convolutional decoder or an equalizer. The new design also applies to coherent optical communications where delay in the loop limits the laser line width. We provide an easy to use complete design procedure for second-order loops. We also introduce a design procedure for higher-order loops for near-optimal performance. We show that using the traditional second-order loop is suboptimal when there is a delay in the loop, and also show large improvements, either in the amount of allowed delay or the phase error variance in the presence of delay.

We propose a new coarse frequency offset estimation algorithm in orthogonal frequency-division multiplexing systems, which performs robust operation in the presence of a symbol timing offset within an allowed range. The effect of a symbol timing offset on estimating a coarse frequency offset is analyzed, and the coherence phase bandwidth according to the allowed symbol timing offset is introduced. The proposed algorithm adapts the concept of the coherence phase bandwidth for the purpose of weakening the effect of the symbol timing offset. Index Terms--Frequency offset, OFDM, synchronization.

Code-division multiple access (CDMA) is a multiplexing technique where a number of users simultaneously access a transmission channel by modulating and spreading their signals with preassigned codewords. This paper studies the performance of CDMA signals with orthogonal (Walsh-Hadamard) codewords and synchronization errors smaller than the chip time. Two high-order modulation techniques, M-level quadrature amplitude modulation (M-QAM) and M-level phase-shift keying (M-PSK) are compared with respect to bit-error rate (BER). The results are especially important for the return channel of cable TV networks and summarized as follows. * Synchronization errors between transmitters lead to interference noise, whereas synchronization errors between the transmitter and the receiver lead to a decreased amplitude of the received user signal. Both effects have significant impact on the system performance. * Closed expressions are obtained for the BER of a CDMA signal with M-PSK and M-QAM with a given maximum synchronization error. * The higher the modulation order, the more sensitive the system gets for synchronization errors. * The BER is highly dependent on the assigned codewords out of the Walsh-Hadamard code set. * The BER performance of M-QAM outperforms that of M-PSK. Index Terms--CDMA, digital communication, high-order modulation, spread spectrum, Walsh-Hadamard codewords.

This paper details on the design of OFDM receivers. Special attention is paid to the OFDM-specific receiver functions necessary to demodulate the received signal and deliver soft information to the outer receiver for decoding. In Part I of the paper, the effects of nonideal transmission conditions have been thoroughly analyzed. To show the impact of the synchronization algorithms--which are most critical in OFDM--on system performance and complexity we consider the design of a complete receiver consisting of symbol synchronization, carrier/sampling clock synchronization and channel estimation. The performance of the algorithms is analyzed and a qualitative estimate of the resulting complexity is given. This allows to draw conclusions concerning the achievable system performance under realistic complexity assumptions. Index Terms--OFDM, receiver design, synchronization.

Frame synchronization is traditionally achieved by inserting a given sequence periodically into the data stream. Here, a new method of synchronization is introduced in which the synchronization sequence is not contiguous, but is interleaved with the data symbols. The properties of these distributed sequences are investigated, and requirements are specified for detectibility of these sequences when inserted in random data. Methods for constructing and searching for good sequences are presented. It is demonstrated that this new method reduces the probability of false synchronization over traditional methods in the presence of additive white Gaussian noise. Index Terms--Correlation, frame synchronization, Gaussian channels, maximum-likelihood detection, sequences, signal detection, synchronization.

In this paper, we focus on the use of Cramer--Rao-like bounds for the estimation of a deterministic parameter in the presence of random nuisance parameters. As a working example, we consider the problem of estimating the carrier frequency offset affecting a linearly modulated waveform received through a Rician fading channel. We calculate the relevant Cramer--Rao bound (CRB) and also a set of Cramer--Rao-like bounds recently proposed in the literature, among which we mention the hybrid CRB, the modified CRB and the Miller--Chang bound. An extension of the latter bound is also provided. The relative merits of these bounds are discussed, both in terms of their tightness and ease of calculation. A few useful inequalities involving the above bounds are also established. Index Terms--Cramer--Rao bounds, frequency estimation, Rician channel, synchronization.

The performance of a binary-quantized data-transition tracking loop symbol synchronizer operating in cochannel interference (CCI) and noise is examined for both binary phase-shift keying and quaternary phase-shift keying modulations. The CCI can be included in a previously known Markov chain model without increasing the number of states in the chain, but the cost of computing the transition probabilities increases exponentially with the number of interferers. It is shown that a first-order Markov model and simulation are in close agreement for some important applications. The effect of interferer epoch on the degradation caused by the interference is investigated and the degradations in performance caused by CCI and by Gaussian noise are compared. Index Terms--Cochannel interference, digital circuits, phase-locked loops, synchronization, timing, tracking loops.

Direct-sequence code-division multiple access is emerging as a potential multiple-access communication scheme for future digital wireless communications systems. Such wide-band systems usually operate in a frequency-selective fading channel that introduces intersymbol interference and thus potential performance degradation. Previously proposed subspace-based blind channel identification algorithms, which provide estimates of channel parameters for effective equalization, suffer from high numerical complexity for systems with large spreading gains. In this paper, it is shown that, through the use of matched filter outputs, reduction in numerical complexity can be obtained. The complexity reduction is considerable when the channel length is small and the system is moderately loaded. The results show that the new algorithm suffers a slight performance loss. Although the employed matched filter outputs do not form a set of sufficient statistics for the unknown channels, the difference between the matched filter outputs and the sufficient statistics becomes negligible for large observation lengths and the asymptotic normalized Fisher information does not change. Performance is evaluated through simulations, the derivation of a tight approximation of the mean-squared channel estimation error, and through comparisons to the Cramer-Rao bound for the estimation error variance. It is shown that the approximation of the mean-squared error can be obtained in terms of the correlation of the spreading codes and the channels. This representation of the error supplies a tool for investigating the relationship between performance and spreading sequence correlations. Index Terms--Blind channel identification, dimension reduction, DS/CDMA, multiuser systems, perturbation analysis, spread-spectrum systems, subspace methods, sufficient statistics, synchronization.

The performance of a coherent delay-locked tracking scheme for direct-sequence/spread-spectrum systems using half-sine or triangular chip waveforms for early and late despreading sequences is analyzed. The effect of band-limiting on the received signals is considered. Mean time to lose lock (MTLL) and root mean square (rms) tracking error of the delay-locked loop (DLL) are compared with that of a conventional DLL which uses rectangular chip despreading waveforms. Linear and nonlinear (based on the renewal process approach) analyses are employed to evaluate the performance of the DLL. Results show that the use of either the half-sine or triangular chip waveform reduces the rms tracking error and increases the MTLL considerably when the early-late spacing is approximately between 0.7-1.3 chip times. The results apply in particular to the commonly used DLL using one chip early--late spacing. Computer simulation for band-limited signals confirms the analytical results. The use of these despreading chip waveforms also reduces tracking offset in multipath environments. Index Terms--Chip waveforms, delay-locked loop, direct-sequence spread spectrum, PN code tracking, synchronization.

In order to construct the optimal decorrelator for a quasi-synchronous (QS) code-division multiple-access receiver, it is necessary to estimate the user delays. However, in a QS system, it is shown that delay estimation is equivalent to estimating a parameter vector which is related linearly to the received signal. An exact maximum-likelihood (ML) solution using alternating maximization is first developed for joint estimation of the user delay vectors and amplitudes. Suboptimal recursive least squares (RLS) and least-means square algorithms are then presented, which approximate the true ML solution. It is shown that the optimal decorrelator and demodulated symbols are also generated by the RLS algorithm. Simulation and analytical results are presented for the bit-error rate performance of the adaptive receiver, and the averaged squared error of the estimated delay vectors is compared with the Cramer-Rao bound.

In this paper, we compare performance of several multiuser detectors for differentially encoded data combined with simple, disjoint, decision-directed Kalman channel estimators over flat Rayleigh fading channels. Simpler detectors with noncoherent differential detection are also compared. Different performance trends relative to the case of perfect channel estimation are observed. We find that in the presence of channel mismatch, the linear decorrelator is the most robust detector in terms of the bit-error rate and the near-far resistance. Parameter adjustment for fading channel modeling and estimation in the decision-directed mode are also discussed. Index Terms--Code-division multiple-access, differential phase-shift keying, fading channels, Kalman filtering, multiuser channels.

Because the conventional chaos masking communication scheme is vulnerable to dynamics-based and identification-based attacks, in this paper, we develop a robust digital secure communication scheme by exploiting sporadic coupling synchronization of Lorenz system. The new scheme can not only withstand the attacks effectively but also improves the frequency efficiency and noise resistibility, etc. The secure property and system performance are theoretically analyzed. Computer simulations are performed to illustrate the effectiveness and efficiency of our new scheme. Index Terms--Chaos, digital communication, synchronization, security.

In this paper, a data-aided frequency and symbol synchronization scheme for M-QAM OFDM signals is suggested. At first, the phase discriminator (PD) and frequency discriminator (FD) for, respectively, the symbol and frequency synchronization loop are described. Second, the transfer function and design parameters of the loop filters are provided. The acquisition and tracking performance of the synchronization system are evaluated using simulation for a 4-PSK signal constellation. The cases of additive white Gaussian noise (AWGN) and frequency and time selective multipath Rayleigh channels are separately tested. Index Terms--Frequency synchronization, multipath channel, OFDM, symbol synchronization.

This paper presents a code synchronization scheme--the time-window control loop (TWCL)--for postdetection RAKE receiver systems that offers the path-diversity effect. Laboratory test results from a postdetection RAKE receiver system with TWCL under a frequency-selective fading environment are introduced. RAKE combining methods, a simple integral demodulation method within a time window, and a recursive least squares (RLS) demodulation method within the time window that uses the RLS algorithm to realize maximal ratio combining, are investigated. It is found that TWCL performance with delay paths (double-spike frequency-selective Rayleigh-fading model) surpasses the performance of both acquisition and tracking without delay paths (nonfrequency-selective Rayleigh-fading model) because of the path-diversity effect. The laboratory results show that TWCL provides good acquisition and tracking performance under the fading conditions experienced with maximum Doppler frequencies of 5 or 80 Hz. It is also shown that the signal energy per bit to noise-power spectral-density ratio ([E.sub.b]/[N.sub.0]) required to achieve the bit error rate (BER) of [10.sup.-3] under the double-spike frequency-selective Rayleigh-fading model is reduced by 10 dB compared to the nonfrequency-selective Rayleigh-fading model. The difference in path-diversity effect between the two RAKE combining methods is only 1 dB. From a structural viewpoint, the simple integral demodulation method supports miniaturization and high-speed data-transmission. Index Terms--CDMA, code synchronization, multipath fading, path diversity, RAKE receiver.

Optimal synchronization of two identical chaotic systems coupled in a drive/response manner is considered in the paper. We derive a relationship between the mean square synchronization error and the coupling parameters in the presence of noise. By minimizing the mean square synchronization error with respect to the coupling parameters, an optimal synchronization, which minimizes the synchronization error between the drive and response systems, can be achieved. It is shown that the optimal coupling parameters depend on both the global and local Lyapunov exponents of the chaotic drive system. However, they are independent of the noise power. We apply this approach to design optimal synchronization for various chaotic systems. The optimal design is then applied to chaotic communication and it can recover the information signal efficiently. Simulations show that our theoretical results are in good agreement with the numerical analysis. Index Terms--Chaos, optimal design, synchronization.

1. Introduction Traditionally, the theory of interprocess synchronization has centered around the notion of mutual exclusion: ensuring that only one process at a time is allowed to modify complex shared […], The "wait-free hierarchy" provides a classification of multiprocessor synchronization primitives based on the values of n for which there are deterministic wait-free implementations of n-process consensus using instances of these objects and read-write registers. In a randomized wait-free setting, this classification is degenerate, since n-process consensus can be solved using only O(n) read-write registers. In this paper, we propose a classification of synchronization primitives based on the space complexity of randomized solutions to n-process consensus. A historyless object, such as a read-write register, a swap register, or a test&set register, is an object whose state depends only on the last nontrivial operation that was applied to it. We show that, using historyless objects, Ω([square root of n]) object instances are necessary to solve n-process consensus. This lower bound holds even if the objects have unbounded size and the termination requirement is nondeterministic solo termination, a property strictly weaker than randomized wait-freedom. We then use this result to relate the randomized space complexity of basic multiprocessor synchronization primitives such as shared counters, fetch&add registers, and compare&swap registers. Viewed collectively, our results imply that there is a separation based on space complexity for synchronization primitives in randomized computation, and that this separation differs from that implied by the deterministic "wait-free hierarchy." Categories and Subject Descriptors: F.1.2 [Computation by Abstract Devices]: Modes of Computation-parallelism and concurrency; F.1.3 [Computation by Abstract Devices]: Complexity Measures and Classes--complexity hierarchies Gerneral Terms: Algorithms, Theory Additional Key Words and Phrases: Consensus, lower bounds, space complexity

Abstract. Wait-free implementations of shared objects tolerate the failure of processes, but not the failure of base objects from which they are implemented. We consider the problem of implementing shared […]

As discussed in the above paper, B-ISDN continuous bit-rate services using the synchronous residual time stamp method may be suspected to suffer from excessive jitter. However, this is not true even if a high-frequency offset shifts the stuff ratio to unfavorable stuff ratio causing high-jitter amplitude. But there is another, still open, synchronization issue.

The BER analysis in [1] is based on a Gaussian approximation. We derive a combinatorial BER analysis and an improved Gaussian approximation for synchronous optical code-division multiple-access (S/CDMA) with prime codes. This analysis is shown to be in good agreement with the Gaussian approximation as well as with simulation results.

We consider initial timing acquisition in DS-CDMA when propagation is affected by multipath and fading and were the base-station broadcasts a synchronization pilot signal in the form of bursts of modulated chips transmitted periodically and separated by long silent intervals. Subject to certain simplifying assumptions we derive the maximum-likelihood (ML) estimator by solving a constrained maximization problem. Our ML timing estimator has constant complexity per observation sample. The relation to other estimation methods is addressed, and performance comparisons are provided by simulation. The proposed estimator yields good performance independently of the multipath-intensity profile of the channel, provided that the delay spread is not larger than a given maximum spread. Moreover, our estimator is fairly robust to the mismatch in the fading Doppler spectrum and provides good performance for both fast and slow fading.

Pulse stuffing and pointer adjustment are necessary for clock rate adaptation in synchronizers used in synchronous digital transmission networks (e.g., SDH/SONET). The method of adaptive threshold modulation (AT-Mod) may be used in the synchronizer to reduce the resulting waiting time jitter. This paper in detail addresses the performance of the method of AT-Mod technique. Using a nonlinear model to represent the thresholder, the paper provides mathematical expressions for the filtered jitter power resulting from AT-Mod in the synchronizer. The accuracy of using such a nonlinear technique is also demonstrated. Furthermore, the possibility of improving the AT-Mod circuit for further jitter reduction is discussed. Because the improved AT-Mod circuits are not always guaranteed to be stable, a stability analysis is presented. The impact of input noise on the filtered jitter is also investigated in the paper. Index Terms--Digital communications, jitter, SONET, synchronization, synchronous digital hierarchy (SDH).

A tracking-mode receiver for asynchronous direct-sequence CDMA is presented based on the extended Kalman filter (EKF). The EKF jointly estimates the delays and multipath coefficients of the received CDMA waveform, and provides a modified minimum mean-square error (MMSE) estimate of the user data (MMSE-EKF). In order to obtain a practical algorithm, each user signal is tracked individually, with the remaining users modeled as colored Gaussian noise. However, the EKFs are coupled through the multiple access interference (MAI) covariance estimates. In order to obtain meaningful performance measures, approximate worst-case undesired user delays that minimize the desired user SNR and delay estimation Cramer-Rao bound are obtained. It is shown that such worst-case delays can be efficiently computed using the alternating maximization (A-M) algorithm. The resulting bit error rate (BER) performance of the MMSE-EKF tracking receiver is evaluated through a combination of simulation and analysis. The mean-time to lose lock (MTTLL) for a genie-aided EKF delay estimator is also obtained using the A-M computed delays. Index Terms--CDMA, synchronization, spread spectrum, Kalman filter.

Fault-tolerant distributed computers need the basic property of independence to achieve security, but distributed communications networks are loosely synchronized, with messages at a given round relying on other, possibly malicious players in the same round. A new constant round protocol is described for simultaneous broadcast.

In some private mobile radio/public access mobile radio (PMR/PAMR) applications, there is a stringent need for high-coverage locations probabilities. A spectrally efficient approach in this case is the use of several radio transmitters operating in simulcast mode. There have been several analog mobile radio systems working in this way up to now, but less is known about the performance of digital trunked radio systems operating in simulcast mode. In this paper, predicted digital Trans-European Trunk RAdio (TETRA) system performance results operating in quasi-synchronous mode are presented. These results were obtained by simulation of such a system under a wide range of operational conditions. A comparison is also presented with the European analog standard MPT 1327 currently in operation. It has been concluded that quasi-synchronous techniques well known in analog PMR/PAMR can also be successfully used in digital PMR/PAMR applications. Index Terms - MPT 1327, overlap coverage areas, simulcast, TETRA.

In a digital communications system, data are transmitted from one location to another by mapping bit sequences to symbols, and symbols to sample functions of analog waveforms. The analog waveform passes through a bandlimited (possibly time-varying) analog channel, where the signal is distorted and noise is added. In a conventional system the analog sample functions sent through the channel are weighted sums of one or more sinusoids; in a chaotic communications system the sample functions are segments of chaotic waveforms. At the receiver, the symbol may be recovered by means of coherent detection, where all possible sample functions are known, or by noncoherent detection, where one or more characteristics of the sample functions are estimated. In a coherent receiver, synchronization is the most commonly used technique for recovering the sample functions from the received waveform. These sample functions are then used as reference signals for a correlator. Synchronization-based coherent receivers have advantages over noncoherent receivers in terms of noise performance, bandwidth efficiency (in narrow-band systems) and/or data rate (in chaotic systems). These advantages are lost if synchronization cannot be maintained, for example, under poor propagation conditions. In these circumstances, communication without synchronization may be preferable. In Part I, the theory and operation of conventional communications systems were surveyed and possible fields of application of chaotic communications were identified. In Part II, the theory of conventional telecommunications is extended to chaotic communications, chaotic modulation techniques and receiver configurations are surveyed, and chaotic synchronization schemes are described. In Part III, examples will be given of chaotic communications schemes with and without synchronization, and the performance of these schemes is evaluated in the context of noisy, bandlimited channels. Index Terms - Chaos, communication systems, digital communication, digital modulation, spread spectrum communication.