Signal mapping optimization of 16-QAM for non-linear mode operation

Performance of 16-QAM systems operating in the nonlinear mode of high-power amplifiers (HPAs) is analyzed. Three types of HPAs, such as the Hughes TWTA (traveling-wave-tube amplifier), Siemens linearized TWTA, and Fujitsu linearized GaAs FET, are considered. A baseband signal mapping technique is proposed which compensates for system impairments caused by AM/AM and AM/PM nonlinearities of HPAs. The suboptimally mapped 16-QAM used enables the HPAs to operate close to saturation at significantly reduced CNR (carrier-to-noise ratio) degradation. In conjunction with a novel signal constellation at the transmitter, an adaptive-decision threshold detector is applied at the receiver to improve the system performance. As an illustrative result, the CNR degradation of the proposed system operating at 0.9 dB OBO (output backoff) of Siemens TWTA is 1.9 dB at P(e)=10/sup -6/. This corresponds to 7 dB or even higher CNR improvement compared to a conventional 16-QAM system.<>