网格编码调制(TCM)与脉冲幅度调制(PPM)相结合的T-PPM 编码具备了双方在纠错能力和发射功率效率方面的优点，在空间光通信中有广阔的应用前景。而传统的T-PPM 编码调制技术在映射方面存在缺陷，制约了系统的可靠性。为改善大气湍流光通信系统的误码性能，提出一种适用于单载波弱湍流空间光信道的改进型T-PPM 技术。这种方法采用格雷映射取代传统TCM 中的“子集划分”映射，降低接收时的误判可能性。仿真结果表明，格雷码映射能显著改善误码性能；在误码率为10-3 处，格雷码映射相比子集映射提高了约0.2 dB 增益，有效改善系统的差错性能；在1 ps 和2 ps 展宽量条件下误码率(BER)分别减少至传统方案的约1/2 和4/5，从而验证了格雷码映射的优越性。

T-PPM code combines trellis coded modulation (TCM) and pulse amplitude modulation (PPM). It has both advantages in error correction and power efficiency of emission signal, and it has broad application prospects in space optical communications. The traditional T-PPM modulation and coding has shortage in mapping steps which limits the reliability of the system. In order to improve the bit error performance of atmospheric turbulence optical communication system, an improved T-PPM technology suitable for singlecarrier weak turbulence is proposed. It uses Gray code mapping instead of the traditional“subset”mapping of TCM to reduce the possibility of error decision in receiving. Simulation results show that Gray code mapping can obviously improve bit error performance; at the bit error rate (BER) level of 10-3, compared with“subset” mapping, the Gray code mapping increases coding gain by about 0.2 dB; at pulse expansion level (1 ps and 2 ps), the Gray code mapping scheme can reduce the BER to about 1/2 and 4/5 of that of conventional “subset” mapping scheme, thus proving the priority of Gray code mapping.