Abstract
In this paper, an adaptive polarization-QAM modulation (APQAM) scheme is proposed to improve the power amplifier (PA) energy efficiency (EE) in orthogonal frequency division multiplexing (OFDM) systems. Since we found that polarization modulation (PM) has better EE performance than QAM in the nonlinear region of PA while QAM outperforms PM in the linear region, we can combine these two modulation schemes together and adjust their modulation orders adaptively to obtain higher EE performance. First, a truncated Taylor series model is used to represent PA nonlinearity, so the effects of both the in-band distortions and the out-of-band emissions on the OFDM signals can be analyzed. Second, two APQAM algorithms are investigated in both AWGN channel and depolarization channel with power-dependent loss. Furthermore, symbol error rate performance of the APQAM scheme is obtained in different channel conditions, and the influence of channel estimation error is analyzed to testify the robustness of the proposed APQAM scheme. Simulation results show that, if perfect channel state information is obtained, the EE of the proposed scheme is nearly 50% more than that of the traditional QAM scheme in OFDM systems.
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