Said Moridi

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

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. >

Analysis of Four Decision-Feedback Carrier Recovery Loops in the Presence of Intersymbol Interference

The performance of four decision-feedback carrier recovery techniques is evaluated in the presence of additive noise and intersymbol interference (ISI). For QAM signal constellations, a closed-form expression is given for the phase jitter variance (PJV) of each loop, and the loop tracking performance is examined. The analytic results are then computed in the case of a 16 QAM digital radio system subjected to multipath fading. Two cases are considered: in the first case no countermeasure techniques are used against selective fading, while in the second case a three-coefficient decision-feedback equalizer (DFE) is used. Computer simulations using a pseudorandom sequence to estimate loop performance are also reported which support the theoretical results.

Baseband Equalization and Carrier Recovery in Digital Radio Systems

The influence of adaptive baseband equalization on decision-feedback carrier recovery performance is investigated. First, it is shown that the delay introduced by the equalizer in the carrier recovery loop does not have basic consequences on system performance. Second, the interaction between the two adaptive circuits is studied for four different equalizer adaptation algorithms. It is found that algorithms based on the zero-forcing criterion are unstable unless they are used with an appropriate constraint. With gradient-type algorithms based on the minimum mean square error criterion, the interaction leads to an improved carrier acquisition performance at the expense of a larger phase jitter. It is therefore concluded that even with these latter algorithms it is generally worthwhile to avoid the interaction by forcing the imaginary part of the equalizer reference tap to zero. The theoretical analysis (carried out with a single complex tap equalizer) is supported by experimental results and computer simulation.

Minimum Mean-Square Error Timing Recovery Schemes for Digital Equalizers

Two algorithms are presented for optimum timing recovery in digitally implemented equalizers. The first one is a polarity-type algorithm based on the conventional minimum mean-square error criterion. A theoretical analysis is made to characterize the algorithm phase detector and evaluate its steady-state phase jitter variance. Influence of various channel and system design parameters on the algorithm performance is illustrated using phase jitter probability densities obtained by means of computer simulations. Interaction of the algorithm with decision-directed carrier recovery is also examined. It is shown that interaction with carrier recovery may considerably degrade the timing acquisition performance, and a second algorithm is then presented which eliminates this interaction. The second algorithm is based on the minimization of a modified mean-square error criterion which provides a measure of the intersymbol interference, independently of the carrier phase. Decision-directed timing and carrier recoveries are thus decoupled and the system startup period is considerably reduced. Phase detector characteristic and steady-state jitter performance of the second algorithm are evaluated by analytical means and computer simulations, as in the first algorithm.