High order quadrature amplitude modulation (QAM) can improve spectral efficiency effectively

but it is very sensitive to phase errors.An asynchronous double layered modulation format is proposed to achieve nearly the same spectral efficiency as conventional high order QAM but with lower modulation orders in transmitter and lower sensitivity to phase errors.A closed-form formula for the symbol error rate (SER) of the proposed asynchronous double layered QAM format together with conventional QAM format is derived under flat Rayleigh block fading channel at fixed phase errors.Both the theoretical analysis and the simulation results show that the performance degradation of the proposed asynchronous double 256-QAM forma

t is just 0.3 dB at the SER of 10

-1

as the phase error increased from 0 degree to 0.9 degree.In contrast

the performance degradation of the conventional 4096-QAM format is up to 11 dB.