U. Mengali

GLRT receivers for UWB systems

This letter investigates generalized-likelihood-ratio-test (GLRT) detectors for ultra-wideband (UWB) impulse radio systems employing two alternative signaling schemes. One is the so-called transmitted reference method where, in each time frame, a reference pulse is transmitted prior to the data pulse. The other is the differential transmitted reference scheme, wherein the data pulse received in the previous frame is used as a template waveform in the current frame. The two detectors are compared in terms of bit error rate performance.

Joint Phase and Timing Acquisition in Data-Transmission

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.

Jitter Accumulation in PAM Systems

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 Chapmans 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.

A Self Bit Synchronizer Matched to the Signal Shape

A suboptimum self bit synchronizer is considered that matches the signal shape, i. e., operates making a clever use of the knowledge of the signaling waveform. The noise performance of this system is analyzed for high signal-to-noise ratios, and an expression is obtained for the variance of the timing errors. This variance is compared with that of systems proposed by other authors on the basis of equal closed-loop noise bandwidth and equal signal shape. It is found that the use of this matched synchronizer enables appreciable timing jitter reductions.

Timing extraction in optical transmissions

The variance of the jitter in a timing recovery circuit for fibre optical digital transmission is computed. The timing circuit is simply a narrow-band filter, tuned to the pulse repetition frequency. It is found that, if the time dispersion of the fibre is limited, this circuit provides satisfactory timing information.

Timing recovery for UWB signals

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.

Synchronization of QAM Signals in the Presence of ISI

A data-aided carrier reconstruction method is described for quadrature amplitude modulation (QAM) data communications over narrow-band transmission channels. Previous analyses of this problem have assumed an infinite bandwidth of the communication link, so that negligible intersymbol interference (ISI) resulted. On the contrary, when ISI takes place, a different structure must be used which operates on sampled data. The performance of this structure is expressed analytically as a function of 1) the statistics of the message symbols, 2) the characteristics of the channel, and 3) the parameters of the tracking system. Excellent agreement has been found between the theory and the results of a computer simulation.

Performance Limits on Ranging with Cognitive Radio

Cognitive radio is a promising paradigm for efficient utilization of the radio spectrum due to its capability to sense environmental conditions and adapt its communication and localization features. In this paper, the theoretical limits on time-of-arrival estimation for cognitive radio localization systems are derived in the presence of interference. In addition, an optimal spectrum allocation strategy which provides the best ranging accuracy limits is proposed. The strategy accounts for the constraints from the sensed interference level as well as from the regulatory emission mask. Numerical results are presented to illustrate the improvements that can be achieved by the proposed approach.

Timing recovery in optical data transmission

The problem of timing recovery in optical data communication is considered using the maximum likelihood (ML) estimation. Assuming negligible intersymbol interference, the structure of the ML estimator is first obtained and some approximations of it are suggested both in the form of decision-directed tracking loops and in the form of data-independent trackers. Comparisons are made with other synchronization schemes discussed in the literature.

Limiting the Impulsive Noise in a PLL

This paper analyzes the operation of a phase-locked loop (PLL) preceded by a bandpass limiter (BPL) in the presence of impulsive noise. It is shown that the effect of the limiter consists essentially in a change of the statistics of the pulse strengths of the noise, so that the behavior of a BPL + PLL can be deduced from that of a simple PLL by suitable adjustment of the noise model. It is found that the limiter greatly enhances the PLL performance by reducing both the phase-error variance in the loop and the probability of cycle slippage. Finally, the design of the filter of the BPL is discussed, resulting in the conclusion that the best results are obtained by using a singletuned RLC circuit.