光通信中双幅度脉冲位置调制的功率谱分析

Spectral Characteristics of Dual Amplitude Pulse Position Modulation in Optical Communications

在研究数字脉冲调制时,对调制方式的功率谱分析十分必要。详细推导了随机双幅度脉冲位置调制(DAPPM)序列的功率谱密度,并分别对连续谱和离散谱进行了分析。数值结果表明,DAPPM的功率谱形状类似于一般的抽样函数,并且包含直流分量,幅度因子α只影响功率谱的幅度,不影响功率谱的形状;时隙基频分量存在与否与脉冲占空比有关;时隙基频幅度强弱依赖于调制阶数和脉冲占空比。当占空比为0.5时,时隙基频分量最大,调制阶数每增大1,幅度衰减约5 dB。同时指出,当调制阶数较大时,随机DAPPM序列难以直接提取时隙基频分量。

It is necessary to analyze the power spectral density of modulation mode in optical communications.A detailed derivation of power spectral density of the dual amplitude pulse positiion modulation（DAPPM） scheme is presented and the continuous spectrum and discrete spectrum of DAPPM spectrum are analyzed respectively.Numerical results show that the spectral profile resembles a general sinc envelope shape,which contains direct component.The amplitude coefficient α has nothing to do with shape of continuous spectrum,but the amplitude of continuous spectrum.The amplitude of both components and the location of the slot frequency component largely depend on the modulation level and the pulse duty cycle.The maximum power level of slot frequency components is achieved when pulse duty cycle is 0.5,and decreases linearly at a rate of about 5 dB with the increase of modulation level.It is also shown that the slot frequency is hard to pickup from random DAPPM sequence at large modulation level.