Some Facilities to Improve Existing Microwave Digital Radio Systems by Means of Predistortion and Precoding Methods

We describe three methods to improve existing 16-QAM transmission systems. The first one suppresses the amplitude of quaternary baseband symbols during a certain part of its duration to get a simple digital predistortion instead of the usual analogue x/sin(x) distortion. Another predistortion method compensates for the stationary distortion of a complete individual transmission channel by means of predistorted partial symbol amplitudes determined once -and corresponding to the desired envelope of predistortion- with the help of the theory of non-linear optimization or by empiric experimental methods. Finally, a new correlative coding method avoids jumps of more than one level height between consecutive quaternary symbols. The spectra of the transmitted signals are compressed down to lower frequencies. Decrease in jitter sensitivity, fading influence and other improvements can be shown by computer simulations and hardware measurements.