The performance of equalization techniques for digital radio systems in the presence of sinusoidal and modulated interference signals is investigated. Linear and nonlinear equalizers, as well as synchronous and T/2-spaced structures, are considered. It is shown that synchronous decision-feedback equalizers are powerful countermeasure devices for radio channels affected by both selective fading and sinusoidal interferers, and that both T/2-spaced linear and nonlinear equalizers can provide a significant improvement of the adjacent channel interference margin, whether the channel is selective or not, especially in the case of multicarrier interleaved frequency arrangements.>

A simple method for calculating the coefficients of a digital transversal filter when cascaded with analog filters is proposed. The optimization criterion takes into account both the spectral characteristics of the signal and the residual amount of intersymbol interference at the decision device input. In contrast with standard methods no a priori constraints are applied to the filter in the time or in the frequency domain.>

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.

The performance of adaptive decision feedback equalization applied to high bit rate digital radio systems in the presence of multipath propagation is analytically investigated. Minimum phase (MP) and nonminimum phase (NMP) type fades as well as the transition periods between these two states are considered. Insight is given into the IF recovered timing epoch and its consequences on the decision feedback equalizer (DFE) behavior, especially during transition periods. Required conditions on the DFE structure for ensuring its maximum efficiency are derived and a modified updating algorithm is presented. Finally, dynamic simulation results are reported and compared to theoretical results. They show that the proposed structure is capable of coping with most propagation conditions.

The performance of intersymbol-interference and jitterfree offset-QPSK (IJF-OQPSK) and partial-response (PR) IJFOQPSK modems in a nonlinearly amplified multichannel environment is studied and compared to that of conventional QPSK and OQPSK modems. Power efficiency is maximized by operating the high-power amplifier (HPA) in full saturation. The effect of adjacent-channel interference and flat fade on the performance of illustrative nonlinear satellite systems is investigated. Simulation and measurement results are reported. It is shown that both IJF-OQPSK and PR-IJF-OQPSK modems exhibit a significant performance improvement compared to QPSK and OQPSK modems, in illustrative SCPC and on-board regenerative satellite systems, they provide an improvement of more than 4 dB compared to QPSK.

A new carrier recovery loop which applies to quadrature amplitude shift keying (QASK) and phase shift keying (PSK) modulation schemes is presented. This loop is shown to provide significant advantages compared with those already used. Furthermore, the baseband processing of the demodulated signals allows an easy implementation, especially for high-speed digital modems. Both geometrical interpretation and theoretical justification using estimation criteria are provided. Finally, experimental results are reported.