在深空激光通信中，脉冲位置调制(PPM)是一种常见的调制方式。而传统的多脉冲位置调制(MPPM)方式中每个信号块发送光脉冲的数目是固定的，随着深空探测技术的不断发展，对调制方式的要求也不断提高，需要对MPPM方式进行改进，因此，在MPPM方式基础上，提出了一种新的调制方案（n脉冲XPPM）以提高MPPM的误码率(SER)和带宽利用率。基于n脉冲XPPM的传输特性，推出其传输速率、所需功率和带宽以及SER的表达式，通过与传统的单脉冲位置调制（LPPM）和MPPM比较，得出在相同的平均功率情况下，n脉冲XPPM方法的SER比传统MPPM方法低得多，并且在传输速率以及所需带宽方面，n脉冲XPPM也优于其他两种PPM方式。室内仿真实验表明，在通信距离大约为4×1011 m的地球至火星深空通信系统中，采用n脉冲XPPM方式可以使系统的传输速率达到2.746 Mbit/s，带宽利用率也可达到85%左右。

In deep-space optical communications, the pulse-position modulation (PPM) is a common way of modulation. But in the conventional multiple pulse-position modulation (MPPM), the number of transmitted light pulses per signal block is fixed. With the development of deep space exploration technology, the requirement of the modulation mode must be increasing. It is necessary to improve the MPPM to meet the requirements of deep space optical communication. So we propose a new modulation scheme (n pulse XPPM) to improve both the symbol-error rate (SER) and the bandwidth-utilization efficiency. Based on the transmission characteristics of XPPM, the modulation rate, required power and bandwidth are derived and compared with L-level pulse-position modulation (LPPM) and MPPM. At the same average power, the XPPM scheme achieves much lower levels of SER than the ordinary MPPM scheme. And n pulse XPPM has more advantages than the two other PPM ways in terms of the transmission rate and the required bandwidth. Results of the simulated experiment reveal that the transmission rate and the bandwidth-utilization efficiency of system can reach 2.746 Mbit/s and 85% or more, respectively, when the communication distance of deep space optical communication system from the earth to the mars is 4×1011 m using XPPM.