The paper is concerned with the impact of clock frequency offsets on the accuracy of ranging systems based on time of arrival (TOA) measurements. It is shown that large TOA errors are incurred if the transmitter and receiver clocks are mistuned by more than just one part per million (ppm). This represents a serious obstacle to the use of commercial low-cost quartz oscillators, as they exhibit frequency drifts in the range of ± 10 ppm and more. A solution is to estimate first the transmitter clock frequency relative to the receiver's and then compensate for the difference by acting on the receiver clock tuning. An algorithm is proposed that estimates the transmitter clock frequency with an accuracy better than 0.1 ppm. Computer simulations indicate that its use in ranging systems makes TOA measurements as good as those obtained with perfectly synchronous clocks.

Theoretical limits on time-of-arrival (equivalently, range) estimation are derived for multicarrier systems in the presence of interference. Specifically, closed-form expressions are obtained for Cramer-Rao bounds (CRBs) in various scenarios. In addition, based on CRB expressions, an optimal power allocation (or, spectrum shaping) strategy is proposed. This strategy considers the constraints not only from the sensed interference level but also from the regulatory emission mask. Numerical results are presented to illustrate the improvements achievable with the optimal power allocation scheme, and a maximum likelihood time-of-arrival estimation algorithm is studied to assess the effects of the proposed approach in practical estimators.

A joint TOA/AOA estimator is proposed for UWB indoor ranging under LOS operating conditions. The estimator employs an array of antennas, each feeding a demodulator consisting in a squarer and a low-pass filter. Signal samples taken at Nyquist rate at the filter outputs are processed to produce TOA and AOA estimates. Performance is assessed with transmitted pulses with a bandwidth of either 1.5 GHz (type-1 pulses) or 0.5 GHz (type-2 pulses), which correspond to sampling rates of 3 GHz and 1 GHz, respectively. As expected, the estimation accuracy decreases with the pulse bandwidth. Ranging errors of about 10 cm and angular errors of about 1° are achieved at SNR of practical interest with type-1 pulses and two antennas at a distance of 50 cm. With type-2 pulses the errors increase to 35 cm and 3°. Comparisons are made with other schemes discussed in literature.

Recent studies have investigated alternative methods to the realization of transmitted-reference ultra-wideband communication systems. In addition to the conventional scheme, in which data and reference are separated in time, two other methods have been proposed in which the separation is achieved through a frequency shift or via code multiplexing. In a single-user scenario the alternative methods have advantages in terms of error rate and receiver implementation. In this paper the three schemes are compared in a multi-user environment. It is shown that even in the presence of multiple access interference the unconventional solutions are preferable and, in particular, the code multiplexing method has the best performance and is the simplest to implement.

This paper discusses a ranging algorithm appropriate for impulse radio ultra-wideband signals compliant with the IEEE 802.15.4a standard. Three steps are indicated to locate the so-called ranging marker in the received signal. The first is to find the highest peaks in the channel responses to the transmitted pulses. The second consists in measuring the distance of each peak to the starting time of the response. The last one involves the identification of the first symbol of the physical-layer header. Algorithms to accomplish all these tasks are indicated. They are implemented in digital form and are in keeping with a non-coherent receiver structure. The achievable accuracy depends on the sampling rate. For example, at SNR = 20 dB and with a rate of 1 GHz the ranging errors are on the order of 20 cm.

This paper deals with a wireless indoor positioning problem in which the location of a tag is estimated from range measurements taken by fixed beacons. The measurements may be affected by non-line-of-sight (NLOS) errors that must be mitigated. We discuss a maximum likelihood (ML) positioning technique that assumes a realistic model for the range errors and a signature database providing information on the propagation conditions at every hypothesized tag spot. The database can be gathered from knowledge of the service area infrastructure and through pre-measurements. It is given in the form of a map indicating, at any node of a close-mesh grid, the nature (LOS/NLOS) of the link between that node and each beacon. The performance of the positioning algorithm is assessed by simulation and is compared with other methods available in literature. The results show that the proposed technique provides significant improvements and is robust against mismatches between true and assumed values of the parameters in the range error model. Comparisons with the Cramer-Rao Bound are made.

Recent studies have investigated alternative methods to the realization of the transmitted reference scheme in UWB communications. In addition to the conventional system, in which data and reference are separated by a time shift, two other methods have been proposed in which the separation is achieved through a frequency offset (FO) or code multiplexing (CM). Their advantage over the traditional system is both in performance and receiver implementation. In this paper they are compared in a multiple access environment. It is shown that the CM-based scheme outperforms its FO-based counterpart and is simpler to implement.

In traditional transmitted reference (TR) ultrawideband systems the reference component is time-shifted and orthogonal relative to the data-bearing signal. This paves the way to a correlation receiver in which the local template is derived from the incoming waveform using a delay line. As analog delay lines are difficult to implement with current technology, an alternative TR system has recently been proposed in which reference and data components are made orthogonal by a frequency shift rather than a time shift. The resulting receiver has no delay lines and has better performance compared to the traditional scheme. In the present paper we discuss a third way to achieve orthogonality, i.e., by modulating reference and data components with two distinct code sequences. Even in this case the receiver has no delay lines. However, it is simpler to implement and has better performance than the frequency-shift based receiver.

This paper investigates time of arrival estimation via impulse radio ultra-wideband technology. A dense multipath channel is assumed and frequency-selective propagation effects are taken into account that make the individual received pulses different in shape from the transmitted monocycles. In these conditions a conventional correlation-based estimator cannot be used and other schemes must be resorted to. Assuming only an approximate knowledge of the received pulses' duration, a TOA estimator is derived making use of least mean square techniques. The algorithm operates on energy measurements made on a folded version of the received signal. Its performance is investigated by simulation under implementation conditions of different complexity. In particular, sub-Nyquist sampling rates are considered and analog-to-digital conversion with limited resolution is explored. Comparisons are made with other available schemes.

This paper investigates time of arrival estimation via impulse radio ultra-wideband technology. A dense multipath channel is assumed and frequency-selective propagation effects are taken into account that make the individual received pulses different in shape from the transmitted monocycles. Assuming only an approximate knowledge of the received pulses' duration, a time of arrival estimator is derived making use of least mean square techniques. The algorithm operates on energy measurements made on a folded version of the received signal. The estimator performance is investigated by simulation under implementation conditions of different complexity. Comparisons are made with other available schemes.

This paper deals with a novel energy-detection receiver for ultra-wideband systems operating in dense multi-path environments. For binary PPM modulation we propose a detection scheme that operates on signal energy measurements taken on small fractions (bins) of the symbol period. Assuming that the fractional energies of the channel response over those bins can be estimated in some way, we look for the decision strategy that minimizes the error probability. This leads us to a receiver structure that generalizes the conventional energy- detection scheme. The new strategy turns out to be superior to the conventional strategy and its performance is found to improve as the bin size decreases. A simple method is proposed to estimate the fractional energies of the channel response exploiting a training sequence. The impact of the estimation errors on the receiver performance is shown to be marginal.

This paper investigates timing recovery for differential transmitted reference (DTR) ultra-wideband receivers. In DTR systems the symbol energy is distributed over several time slots (frames) and a single pulse is transmitted per frame. The information is conveyed by differentially encoding the pulse polarities. Equal polarities correspond to a bit +1 while alternate polarities correspond to a bit -1. The detector computes the decision statistic by correlating pulses from adjacent frames and summing the contributions from all the frames of a symbol. Synchronization consists in providing accurate timing for the correlation computations. A timing algorithm is proposed requiring a limited amount of circuitry in addition to that needed for detection purposes. No training sequences are needed as the synchronizer operates in a blind mode. Two possible receiver architectures are envisioned, analog or digital. The effects of the timing errors on the receiver performance are investigated by simulation. In general, they are found to be negligible, except when the implementation is digital and the analog-to-digital converter resolution is limited to 1 bit.

This paper deals with energy detection receivers for ultra-wideband systems operating in a dense multipath environment. For binary PPM modulation we propose a novel detection scheme that exploits some information on the channel energy-delay profile. We show that the BER performance of this detector is superior to that of a conventional scheme operating with no knowledge of the channel response. A simple method for estimating the channel delay profile is discussed and it is shown that the impact of the estimation errors on the receiver performance is negligible.

This paper investigates timing recovery for differential transmitted reference ultra-wideband receivers. The detector computes the decision statistic by correlating pulses from adjacent frames and summing the contributions from all the frames of a symbol. Synchronization consists of providing accurate timing for the correlation computations. A solution to this problem is proposed that requires a limited amount of extra circuitry. No training sequences are needed as the synchronizer operates in a blind mode. Two possible receiver architectures are envisioned, analog and digital. The effects of the timing errors on the receiver performance are investigated by simulation.

This paper is concerned with timing recovery for ultra-wideband communication systems operating in a dense multipath environment. Two timing algorithms are proposed that exploit the samples of the received signal to estimate the start of the individual frames with respect to the receiver's clock (frame timing) and the location of the first frame in each symbol (symbol timing). Channel estimation comes out as a by-product and can be used for coherent matched filter detection. The proposed algorithms require sampling rates on the order of the inverse of the pulse duration. Their performance is assessed by simulating the operation of coherent and differential detectors. Their sensitivity to the sampling rate is discussed, and the effects of the multiple access interference are evaluated.

The paper investigates ultra-wideband receivers for applications in sensor and control networks, M-ary pulse position modulation is considered and the detector operates with no knowledge of the channel response. It is shown that the optimum decision rule consists of comparing the signal energies received in the M possible pulse positions in a symbol interval and deciding for that position corresponding to the maximum energy. Simple synchronization algorithms are proposed and their performance is assessed by simulation over realistic multipath channels. The algorithms are non-data-aided (blind) and have a feedback structure in which the error signal is based on energy measurements from the received waveform. The receiver performance degradations due to timing errors are found to be marginal

This paper investigates the estimation of the frame frequency in ultrawideband (UWB) communication systems. An estimation method is proposed that exploits the transmission of a periodic pulse sequence at the frame frequency. The samples of the received waveform are used to compute a cost function that depends on a trial value of the incoming pulse frequency. The location of the maximum provides an estimate of the transmitted frequency. The performance of the estimator is assessed theoretically and is compared to the Cramer-Rao lower bound. It is shown that in certain conditions the estimator achieves the bound at high SNR values. Simulations validate the theory and show the degradations in the estimation performance caused by multiple-access interference. They also give an idea of the estimation accuracy needed in a correlation receiver.

This paper deals with channel acquisition and tracking for the uplink of direct-sequence code-division multiple-access systems. The transmission medium is characterized by multipath propagation, and the goal is to estimate the time-varying channel impulse response (CIR) for a new user entering the system. Channel acquisition is pursued through maximum-likelihood techniques. The resulting scheme may be too complex in some applications, as it requires the online inversion of a large matrix. Therefore, we also consider a simpler solution based on the least-squares (LS) criterion. Channel tracking is performed through weighted LS methods. At each signaling interval, the CIR estimate is updated using data decisions and exploiting the inverse of the interference covariance matrix to mitigate the near-far problem. Performance is assessed by simulation in a scenario inspired by the frequency-division duplexing component of the universal mobile telecommunications system. The acquisition/tracking algorithms are found to be resistant to multiuser interference and suitable for transmissions over fast-fading channels.

The paper is concerned with timing recovery for ultra-wideband communications operating in a dense multipath environment. A timing algorithm is proposed that exploits the samples of the received signal to estimate the start of the individual frames with respect to a local reference (frame timing) and the location of the first frame in each symbol (symbol timing). Channel estimation is inherent in the timing algorithm and can be used for coherent matched filter detection.

Two signaling formats for ultrawide bandwidth indoor communications are compared in terms of bit error rate performance in the presence of multiple access interference. The channel exhibits multipath propagation and its impulse response is either assumed known or is estimated with least squares methods. Computer simulations show that, even with perfect channel knowledge, time-hopping pulse-amplitude modulation is superior to time-hopping pulse-position modulation. The superiority increases with the number of users and becomes substantial in the presence of channel estimation errors. An intuitive explanation of this fact is provided.

The time-division duplex (TDD) component of the universal mobile telecommunications system (UMTS) employs synchronous code-division multiple-access techniques with orthogonal spreading codes to provide protection against cochannel interference. In the presence of multipath propagation, however, the code orthogonality is lost and multiaccess interference is generated at the receiver. In such conditions, an estimate of the channel impulse response is required for reliable detection. In this paper, we propose and compare two pilot-assisted schemes for channel estimation in the downlink of the UMTS-TDD system. Both algorithms also provide estimates of the users' energies, which are needed to perform multiuser detection. Theoretical analysis and computer simulations are used to assess the channel estimation performance in terms of mean-squared errors and bit-error rate. It is shown that the accuracy of the proposed estimators attains the Cramer-Rao bound at intermediate/high signal-to-noise ratios.

We consider the uplink of a direct-sequence code-division multiple-access system and we assume that the base station is endowed with a linear antenna array. Transmission takes place over a multipath channel and the goal is to estimate the channel parameters (path gains and delays) and the directions of arrival of the signal echoes from a user entering the network. We propose an estimator that operates in an iterative fashion and exploits knowledge of the transmitted symbols (training sequence). Compared to other existing schemes, it is simpler to implement as it reduces a complicated multidimensional optimization problem to a sequence of one-dimensional searches. Computer simulations indicate that the proposed scheme is useful even in applications over rapidly varying channels where the training sequence must be short compared with the channel decorrelation time.

Synchronization and channel acquisition for ultra-wideband signals are investigated. The channel impulse response is estimated via two approaches: a least-squares method which ignores channel structure, and a subspace technique which finds channel sparseness and then exploits this structure for final channel estimation. Symbol and frame synchronization is also accomplished using least squares methods. Performance is investigated through the evaluation of mean-squared channel estimation error and probability of error with a RAKE receiver employing the channel estimates for an indoor wireless channel. It is observed that the subspace method exploiting the clustered property of the channel yields the best performance at the expense of complexity.

This paper investigates channel acquisition and tracking for the uplink of a DS-CDMA system. The transmission medium is characterized by multipath propagation and the goal is the estimate the time-varying channel impulse response (CIR) of a new user entering the system. Channel acquisition is pursued through maximum likelihood techniques while channel tracking is performed through weighted least squares methods. At each signaling interval the CIR estimate the inverse of the interference covariance matrix to mitigate the near-far problem. Performance is assessed by simulation in a scenario inspired by the UMTS system.

We consider the uplink of a DS-CDMA system and we assume that the base station is endowed with a linear antenna array. Transmission takes place over a multipath channel and the goal is to estimate the channel parameters and the directions of arrival of the signals from a user entering the network. Maximum likelihood estimation of all these parameters is not feasible as it involves a search over a multidimensional domain. Through suitable approximations we replace the above search by a sequence of mono-dimensional searches. This results in an estimation algorithm of reasonable complexity of third-generation cellular applications. The performance of the algorithm is assessed by simulation in a scenario inspired by the specification of the FDD component of the UMTS standard. It is found that the channel parameters and the directions of arrival can be estimated with accuracy close to the Cramer-Rao bound.

This paper deals with channel estimation in the uplink of a direct-sequence code-division multiple-access system operating in a multipath environment. The parameters of interest are the delays and the complex attenuations incurred by the signal echoes along the propagation paths. We propose an iterative approach for estimating the channel parameters of a new user entering the system. The method is based on the space-alternating generalized expectation-maximization algorithm and exploits a training sequence. In comparison to other estimation algorithms, it reduces a complicated multidimensional optimization problem to a sequence of one-dimensional problems. In addition, it can be effectively used in applications over fast-fading channels. Computer simulations are employed to assess the performance of the proposed scheme. It is found that it is resistant to multiuser interference and has accuracy close to the Cramer-Rao lower bound even with very short training sequences.

This paper investigates channel estimation in the uplink of a DS-CDMA system. The transmission medium is characterized by the presence of multipath components and the parameters of interest are the delays and complex gains of the propagation paths. We propose a new method for estimating the channel parameters of a user entering the system. The method is based on the maximum likelihood criterion and exploits a training sequence. In comparison with other estimation algorithms it has two advantages. First, it reduces a complicated multidimensional optimization problem to a sequence of one-dimensional problems. Secondly, it has better capability to resolve closely-spaced multipath components. Computer simulations are used to assess its performance. It is found that it is resistant to multiuser interference and can be effectively used in third generation mobile cellular networks.

This paper deals with channel estimation in ultra-wideband communications operating in a multipath environment and in the presence of multiaccess interference. The channel parameters are the attenuations and delays incurred by the signal echoes along the propagation paths. Time-hopping modulation with binary symbols is assumed. The estimation method is based on the maximum-likelihood criterion and is applied to two different scenarios: either with known symbols (DA estimation) or with unknown symbols (NDA estimation). The effects of the estimation errors on the performance of a RAKE receiver are assessed by simulation by comparing the receiver bit-error rate with either perfect channel estimates or imperfect estimates as obtained from the proposed algorithms. The results show that the degradations are tolerable as long as the number of users is limited. They also show that the DA method has an edge over the NDA in that it can handle a larger number of users for a fixed degradation. The number of users that can be accommodated in practice is found for some values of the system parameters.

We propose a carrier frequency recovery scheme for the downlink of the time-division duplex (TDD) component of the Universal Mobile Telecommunications System (UMTS). The frequency estimator exploits the midamble of the common control channel and takes into account that the frequency offset allowed in the UMTS-TDD is small compared to the chip rate. Simulations are used to assess the accuracy of the proposed scheme assuming diversity reception and a frequency selective channel. It is found that good performance can be achieved with a limited computational load.

The time division duplex (TDD) component of the Universal Mobile Telecommunications System (UMTS) employs synchronous CDMA techniques with orthogonal spreading codes to provide protection against cochannel interference. In the presence of multipath propagation, however, the code orthogonality is lost and multiaccess interference is generated at the receiver. In such conditions an estimate of the channel impulse response is required for reliable detection. In this paper we propose a method for channel estimation in the downlink of the UMTS-TDD system. In addition to the channel impulse response, the algorithm provides estimates of the users' powers, which may be needed to perform multiuser detection. Theoretical analysis and computer simulations are used to assess the performance in terms of mean squared estimation error. It is shown that the accuracy of the proposed estimator attains the Cramer-Rao bound (CRB) at signal-to-noise ratios of practical interest.

This paper deals with carrier-frequency estimation for burst transmissions over frequency-selective channels. Three estimation schemes are proposed, all based on the use of known training sequences. The first scheme employs an arbitrary sequence and provides joint maximum-likelihood (ML) estimates of the carrier frequency and the channel response. Its implementation complexity is relatively high but its accuracy achieves the Cramer-Rao bound. The second scheme is still based on the ML criterion, but the training sequence is periodic, which helps to reduce the computational load. The third scheme also employs periodic sequences, but its structure comes from heuristic reasoning. Theoretical analysis and simulations are employed to assess the performance of the three schemes.

This paper is concerned with the estimation of two synchronization parameters that play a key role in multi-h continuous-phase modulation receivers-the ordinary symbol timing phase and the so-called superbaud timing phase. The recovery of symbol and superbaud timing is implemented by means of feedforward nondata-aided algorithms. The novelty of the proposed method is that it can be applied to any modulation format, either full or partial response, with binary or multilevel symbols and with arbitrary modulation indices.

We investigate a novel nondata-aided method for jointly estimating timing and carrier frequency offset in MSK-type modulation. The algorithm has a feedforward structure and lends itself to a simple digital implementation. Its estimation accuracy depends on a design parameter that may be varied to trade performance for computational complexity. Setting the parameter to unity yields a synchronization scheme already known in the literature. Computer simulations are used to assess the synchronizer performance on AWGN Rayleigh fading channels with MSK and Gaussian MSK modulation.

Novel symbol-by-symbol differential detection algorithms are proposed for minimum-shift keying signals transmitted over additive white Gaussian noise and frequency-flat Rayleigh fading channels. They are derived as approximations to the maximum likelihood noncoherent detection strategy. Their error performance is assessed by computer simulation and is compared with that of other noncoherent detectors. It is shown that, with fading channels, the new algorithms outperform the traditional methods.

Linearly time-varying fading models are used to investigate differential detection of PSK signals transmitted over frequency-flat fading channels. The structure of the optimal differential PSK detector is derived and an analytical technique is developed to compute its error performance. Results obtained by computer simulation are in excellent agreement with the analytical predictions.

Linearly time-varying fading models are used to investigate the differential detection of PSK signals transmitted over frequency-flat fading channels. The structure of an optimal differential PSK detector is derived and an analytical technique is proposed to compute its error performance. The results obtained by computer simulation are in excellent agreement with the analytical predictions.

In this paper we extend the scalar modified Cramer-Rao bound (MCRB) to the estimation of a vector of nonrandom parameters in the presence of nuisance parameters. The resulting bound is denoted with the acronym MCRVB, where "V" stands for "vector". As with the scalar bound, the MCRVB is generally looser than the conventional CRVB, but the two bounds are shown to coincide in some situations of practical interest. The MCRVB is applied to the joint estimation of carrier frequency, phase, and symbol epoch of a linearly modulated waveform corrupted by correlated impulsive noise (encompassing white Gaussian noise as a particular case), wherein data symbols and noise power are regarded as nuisance parameters. In this situation, calculation of the conventional CRVB is infeasible, while application of the MCRVB leads to simple useful expressions with moderate analytical effort. When specialized to the case of white Gaussian noise, the MCRVB yields results already available in the literature in fragmentary form and simplified contexts.

We compute the error performance of noncoherent detection receivers for FSK signals transmitted over fast frequency-flat Rician fading channels. Linearly time-varying fading models are used to derive closed-form expressions for the error probability of binary FSK signaling. Error bounds are established for the performance of M-ary orthogonal FSK. Simulation results are in excellent agreement with analytical predictions.

We describe an algorithm for joint timing and carrier phase recovery with CPM signaling. The proposed method may be employed with any CPM format and with either full or reduced state detectors. It can be implemented in digital form with a limited computational complexity. Simulation results show that its tracking performance is excellent. However, when operated in its simplest form, it may exhibit false locks during acquisition. In particular, this happens with multilevel partial response formats. A simple solution to false locks is proposed.

Linearly time-varying fading models are used to investigate noncoherent detection of FSK signals transmitted over frequency-flat fading channels. The structure of the optimal noncoherent FSK detector is derived and a novel analytical technique is proposed to compute the error performance of noncoherent FSK detectors in fast fading. Error performance results obtained by computer simulation are in excellent agreement with the analytical predictions.

A simple technique for removing intersymbol interference (ISI) introduced by "linearly frequency-selective" fading channels is presented. The technique involves the optimization of the overall impulse response of the transmit and receive filters and effectively reduces the channel to one which is flat fading. Computer simulation results show that this equalization method works for channels with small delay spreads.

Burst transmission of digital data is employed in several applications such as satellite time-division multiple access (TDMA) systems and terrestrial mobile cellular radio. We propose a new algorithm for carrier frequency estimation in burst-mode phase shift keying (PSK) transmissions. The algorithm is data-aided and clock-aided and has a feedforward structure that is easy to implement in digital form. Its estimation range is large, about /spl plusmn/20% of the symbol rate and its accuracy is close to the Cramer-Rao bound (CRB) for a signal-to-noise ratio (SNR) as low as 0 dB. Comparisons with earlier methods are discussed.

We describe joint carrier phase and timing recovery algorithms for CPM signaling. They may be employed with any CPM format, and with either full or reduced state detectors. Their implementation is fully digital, and involves a limited computational complexity. Simulation results show that these algorithms have excellent tracking performance. When operated in their simplest form, however, they may exhibit false locks. In particular, this occurs with multilevel and partial response formats. A simple solution to the false lock problem is proposed.

Linearly time-varying fading models are used to investigate noncoherent detection of frequency shift keying (FSK) signals transmitted over frequency-flat fading channels. The structure of the optimal noncoherent FSK detector is derived and a novel analytical technique is proposed to compute the error performance of noncoherent FSK detectors in fast fading. Error performance results obtained by computer simulation are in excellent agreement with the analytical predictions.

Symbol timing in continuous phase modulated signals is investigated making use of maximum likelihood estimation methods. Nondata-aided algorithms are proposed with a feedforward structure. They are suitable for digital implementation and can be employed with either full or partial response signaling. Multilevel symbol alphabets are allowed and the modulation index may be arbitrary. The performance is assessed by analysis and simulation. It turns out that full response schemes are comparatively easier to synchronize. Difficulties arise with partial response signaling especially with long frequency pulses.

This paper describes novel detection algorithms for coded and uncoded M-ary phase shift keying (M-PSK) signals transmitted over frequency-flat fading channels. Fading distortion is estimated with per-survivor methods using the sampled outputs of two receive filters. Some of the proposed algorithms do not require information on the fading statistics and are dubbed blind algorithms. The error rate performance of blind and nonblind algorithms is assessed by simulation for uncoded and trellis-coded phase shift keying (PSK) and is compared to the performance of other detection schemes proposed in the literature.

Frequency tracking loops are used in digital links to control the carrier frequency of the received signals. A basic component in such loops is the frequency difference detector (FDD). This paper concentrates on a class of FDDs for use with binary continuous-phase modulation (CPM). Their derivation is based on two ideas: the maximum-likelihood principle for parameter estimation and Laurent's (1986) decomposition of a CPM signal into the sum of linearly modulated components. The proposed FDDs are suitable for digital implementation and their operation is independent of clock recovery. Analysis and simulation are employed to investigate tracking operation and acquisition capability of frequency control loops endowed with such FDDs. Numerical examples are provided for modulation schemes of practical interest such as MSK and Gaussian MSK. The loop acquisition range is found to be fairly large, about 0.5-1.0 times the bit rate.>

Optimum decision methods are employed to discriminate between in-sync and out-of-sync operations in Viterbi decoders when the received data sequence is given in analog form (soft decisions). The exact solution to this discrimination problem, however, appears too complex in many practical cases. A suboptimum algorithm is then proposed that can be implemented as a hardware device, external to the decoder. This algorithm is recognized as a modified version of an earlier technique known as the "reencoding method". Analysis and simulations indicate that the modification produces remarkably good results not only in comparison with the reencoding method but also with previous techniques discussed in literature.>

It is widely known that minimum shift keying (MSK) may be seen as a PAM signaling scheme and that the same is true, albeit approximately, with MSK-like modulations. It is also known (perhaps not so widely) that any binary continuous phase-modulated (CPM) signal may be exactly decomposed into the sum of a few PAM waveforms. In this paper we show that this property extends to multilevel CPM signaling. Features of a PAM decomposition are discussed as a function of the alphabet size, the modulation index, and the frequency response of the system. It is found that, especially with signaling schemes with a long memory, the decomposition has so many terms that it becomes unmanageable. For these cases an approximation is proposed with a limited number of terms.>

This paper describes a design method for the synthesis of digital filters in data communication systems. The coefficients in the transmit and receive filters are optimized to meet specified requirements on (i) out-of-band emission, (ii) intersymbol interference, (iii) adjacent channel interference and (iv) thermal noise immunity.>

Frequency tracking loops are used in digital links to control the carrier frequency of the received signals. A basic component in these loops is the frequency difference detector. This paper concentrates on two detectors that have previously been derived from maximum likelihood estimation criteria. Noise spectra of the detector outputs are computed and the results are used to assess the performance of frequency control loops operating in the tracking mode. Computer simulations are used to validate the theory. The detector implementation in digital form is also addressed and design criteria are provided that lead to structures with a limited computing complexity. Comparisons are made with other frequency detection schemes available in literature.>

We introduce the modified Cramer-Rao bound (CRB) which, like the true CRB, is a lower bound to the error variance of any parameter estimator. The modified CRB proves useful when, in addition to the parameter to be estimated, the observed data also depend on other unwanted parameters. The relationship between the modified and true CRB is established and applications are discussed regarding the estimation of carrier-frequency offset, carrier phase, and timing epoch in linearly modulated signals. Modified CRBs for phase and timing estimation have been already discussed in previous works where it is shown that several practical carrier-phase and clock recovery circuits do attain such bounds. Frequency discrimination, instead, is not so well-represented in the literature and a significant contribution of this paper is the calculation of the modified CRB for frequency estimation. This bound is compared with the performance of some frequency detectors and it is concluded that further work is needed in search of more efficient frequency discrimination methods.>

An approximate method is discussed for maximum likelihood estimation of coded PSK sequences in the presence of an unknown carrier phase. This method makes use of the Viterbi algorithm to find an optimum path in the encoder trellis diagram. Phase estimation is embedded into the structure of the detection algorithm and the branch metrics in the trellis are modified with respect to those employed with known carrier phase. Comparisons with traditional decoding schemes show that the proposed solution has better performance in terms of acquisition time and cycle slips.>

The authors investigate the performance of an automatic frequency control loop operating in a receiver for linearly modulated digital signals. The loop uses a balanced quadricorrelator as a frequency detector. The design of such a detector for optimum loop tracing performance is considered. This problem involves the analysis of the disturbances generated in the quadricorrelator, which are conveniently divided into self-noise and thermal-noise-induced components. It is shown that the quadricorrelator can be designed to make self-noise negligible in the steady state, both with staggered and nonstaggered modulations. This leaves the thermal-noise-induced components as the sole obstacle to loop operation in the tracking mode. The level of such components is then minimized by further selection of the quadricorrelator filters. Numerical examples are worked out to assess the advantages of optimized quadricorrelators over more conventional schemes. It is found that tracking errors can be substantially reduced with no penalty for frequency acquisition times of the loop.>

The minimum-mean-square-error approximation to a generalized minimum-shift-keying (MSK) signal using an offset quadrature phase-shift keying waveform with an elementary pulse, g(t), is considered. It is shown that the optimum shape of G(t), gamma (t), coincides with the average pulse of the generalized MSK signal. Therefore, gamma (-t) is the impulse response of the corresponding average matched fiber.>

An analysis is presented of a balanced quadricorrelator driven by a digital-modulated signal with thermal noise. The aim is to assess its performance as a frequency difference detector. The circuit output is decomposed into a signal term and a disturbance referred to as noise. Exprssions are given for the signal and for the power spectral density of the noise as a function of the various system parameters. Design criteria are provided to minimize the output noise for zero frequency offset. In particular, it is found that at high signal-to-noise ratios the noise level can be made vanishingly small by the proper choice of the quadricorrelator filters. A similar result has been recently discovered for another frequency difference detector, the dual-filter detector. It is shown that a balanced quadricorrelator and a dual-filter detector can be made totally equivalent.>

A rotation detection algorithm for solving the phase ambiguity problem in trellis-coded modulations is proposed. This algorithm is simple to implement and may be an efficient alternative to rotationally invariant codes. Its performance has been investigated by simulation and appears promising.>

The carrier synchronization and symbol timing recovery for two-dimensional trellis-code modulated signals are considered. For many popular carrier and clock synchronizers, and tracking and cycle-slipping performance depends on the trellis code only through the second- and fourth-order moments of the coded channel symbols. For three successively more complicated classes of coded symbol alphabets, simple conditions on the trellis code are derived which are sufficient to yield the same second- and fourth-order moments as for uncoded symbols. For trellis codes satisfying these conditions, the synchronizer performance does not depend on the specific code, and is the same as for uncoded symbols. These sufficient conditions are fulfilled by many codes which have been presented in the literature, some of which are discussed.>

This paper presents an analysis of a synchronization circuit to be used for carrier-phase and symbol-timing recovery in continuous phase modulation systems.M-ary modulation formats with arbitrary pulse shaping and rational modulation indexes are assumed. Circuit performance is expressed in terms of variances of phase and timing errors. Numerical results are provided for some important cases, including minimum shift keying and tamed frequency modulation. Theoretical results are found in good agreement with computer simulations.

A general analysis is provided for the jitter performance of a delay clock regenerator as a function of various system parameters such as modulation format, channel filtering, signal-to-noise ratio, and line delay. The novelty with respect to previous works on the same subject is that we find an expression for the jitter variance of the regenerated clock, while other studies have dealt with an upper bound to such variance. Numerical results have been checked by computer simulations.

Previous authors have found that jitter-free operation is achievable with appropriate pulse shaping when the nonlinearity in the timing recovery circuit is a square-law rectifier. In this paper we consider the general case of an arbitrary nonlinearity and we show that a class of pulses can still be found which provide very good jitter performance. Comparisons are made with previous results.

Computer simulation is employed to assess jitter performance of a clock recovery circuit as a function of the characteristics of the rectifier being used. Several types of rectifiers are compared, some operating at baseband, others at intermediate frequency (IF). It is shown that the best choice between them depends both on the modulation format and on the excess bandwidth factor of the pulse spectrum. In QPSK systems, fourth-law rectifiers outperform the others for rolloff factors up to 0.2 while, for higher values, baseband absolutevalue rectifiers are preferable. In the case of 9QPRS, baseband absolutevalue rectifiers provide jitter reductions of one order of magnitude at high signal-to-noise ratios.

In previous work a jitter model has been used to predict the growth of the timing errors along a chain of PCM regenerators. According to this model each regenerator behaves as a combination of a low-pass filter and a source of noise. The former accounts for the smoothing effect of the time constants of the regenerator, and the latter represents timing errors associated with the clock recovery process. This theory has been brought out under restrictive hypotheses either on the width of the pulses (often assumed very short) or on the form of the nonlinearity in the clock recovery system. In this paper we show that the theory is still valid for any reasonable form of nonlinearity and for pulses wider than the symbol spacing. A method is also given to compute the filter transfer function and the spectral density of the noise.

A method is presented to evaluate the effect of line codes on the timing jitter in a chain of PCM repeaters. The timing circuit in each regeneration section consists of a square-law device followed by a resonant circuit tuned to the transmission rate and by a zero-crossing detector. The analysis applies to a broad class of line codes. The results of the paper indicate that the influence of the code on the jitter accumulation is fairly marginal and that the jitter variance is maximum when the symbols are uncorrelated.

A new approach to the jitter accumulation problem in a chain of regenerative repeaters for pulse amplitude modulation systems is described. In each regeneration section the timing circuit consists of a square-law device followed by a bandpass filter with center frequency close to the transmission rate. It is shown that, from the standpoint of jitter generation and propagation, the timing circuit is equivalent to a low-pass filter driven by the sum of the jitter generated in the previous regenerator plus a noise term. This is actually the jitter propagation mechanism suggested in [6] and referred to as Chapman's model. What makes a difference with the results published in [6] is that in our analysis an expression is given for the spectral density of the noise in the equivalent model in terms of system parameters such as pulse shape, bandpass filter mistuning, offset in the triggering level of the pulse generator, etc. In this way the various jitter sources can be qualified and their effects evaluated.

This paper analyzes the performance of a phase-locked loop driven by the photodetector current in an optical receiver. The variance of the phase-errors of the loop is found as a function of the relevant system parameters. The results are in good agreement with experimental data obtained via computer simulation.

This paper deals with a tracking-loop-type approximation to the maximum likelihood (ML) estimator of the carrier phase in a suppressed-carrier vestigial sideband (VSB) data-transmission system. Two different tracking loops are considered, the one resembling a Costas loop, the other a data-aided receiver (DAR). The performance of these circuits is analyzed in the presence of thermal noise and intersymbol interference, and the results are compared with those relevant to a conventional scheme wherein a pilot tone is used to reconstruct the carrier at the receiver.

This paper deals with the analysis of two different recursive methods forcarrierphaseand timing acquisition in synchronous data-transmission systems. The first method is a practical implementation of the maximum likelihood estimator of the phase and timing parameters; the other isbased on the minimization of the mean square error at the sampling times. In both cases the steady-state performance characteristics are evaluated andthe problem of the coupling between phase and timing adjustments is discussed in view of its effects upon stability and convergence rate of the synchronization algorithm. Finally, the signal design problem isaddressed with the objective of simultaneously attaining adjustment decoupling, jitter minimization and intersymbol interference reduction.

This paper describes a new method for analyzing the acquisition behavior of a second-order generalized tracking system in the absence of noise. In particular it allows closed-form expressions of the pull-in range and of the frequency acquisition time for any type of loop nonlinearity. The results of the present analysis are compared with those already known in the literature and with others obtained via the numerical solution of the system equation.