Long-wave infrared wireless communication has much advantage over short-wave infrared. In this paper, the bit error rate performance of LWIR by DPIM is analyzed. The intensity fluctuations of the optical signal are modeled by log-normal distribution, the intensity attenuations are modeled by empirical formula of fog. The system noise includes generation-recombination noise and thermal noise typically for Photoconductive HgCdTe detector. The numerical results of BER are presented. The results illustrate the BER depends on the turbulence strength, bit rate, optical power and link length.