The performance of a digital fiber optical system employing four-level pulse width modulation (PWM) is considered. It is shown that PWM may be an attractive alternative to pulse amplitude modulation (PAM), especially if a wide band channel is used. The probability density function (pdf) of the timing error is obtained in terms of the pdf of the noise amplitude, taking into account the thermal noise of the amplifier and the signal-dependent shot noise introduced by the photodiode detector. Near-Gaussian optical pulse edges at the receiver are assumed. Using typical system parameters, the variation of bit error rate (BER) with power level, and the effects of different bit rates, mean photodetector gain, and system bandwidths are calculated. It is observed that minimum BER's are achieved at moderate values of mean photodetector gain and that increased system bandwidth results in increased receiver sensitivity.

The use of feedforward correction to reduce nonlinear distortion in a microwave amplifier is investigated, using the Volterra series approach. Expressions are derived for the suppression of main amplifier distortion, in terms of the circuit parameters of the system. It is shown that the gain and phase imbalance in the second interference loop mainly determine the distortion suppression in the feedforward amplifier. An expression is derived for the relationship between amplifier distortion suppression and the degree of gain and phase imbalance in the second interference loop. The effect of the phase imbalance in the second interference loop on the AM-to-AM and AM-to-PM conversion in a feedforward amplifier is discussed; it is shown that the presence of such phase imbalance may cause a substantial difference to exist between the improvement in cross-modulation and that in intermodulation. Further, the gain sensitivity of the feedforward amplifier as a function of gain and phase imbalance is obtained. As an illustrative example, the distortion suppression of a specific IMPATT amplifier with feedforward correction is considered.