NLA-QAM: A Method for Generating High-Power QAM Signals Through Nonlinear Amplification

A generalized technique for generating quadrature amplitude modulated (QAM) signals that permits nonlinear amplification is presented. With this technique a high-power 2^2n -state QAM signal is generated by combining n unfiltered, nonlinear amplified, QPSK signals, n being a positive integer. The specific methods of generating 16- and 64-state signals using this technique are presented. An attractive feature of the technique is that despite significant difference with the conventional method of generating QAM signals, the same straightforward demodulation techniques apply to both, and the P e versus S/N performances are essentially identical. In the 16-state version employing traveling wave tubes (TWT's) or GaAsFET amplifiers, the technique is shown to result in a transmitter output power advantage that is on the order of 5 dB compared to the conventional method. This advantage is achieved, however, at the expense of an additional output power amplifier.