To characterize error performance of pulse position modulation (PPM) with optimal demodulation and digital pulse interval modulation(DPIM) whose variable symbol length makes it difficult to analyze performances,an exact symbol error rate (SER) expression is presented as well as its union-bound about PPM based on maximum-likelihood (ML) detection under turbulence channel,and an approximate SER mathematic model of DPIM grounding on transmitted packet is proposed. Appling pairwise error probability (PEP) theory in conjunction with SER,the bit error rate (BER) is also derived. Simulations show that,union-bound analysis of PPM can yield simple analytical upper bounds and approximations to SER,and the performance of PPM influenced more by turbulence is better than DPIM and on-off keying (OOK). DPIM isn′t suitable for simplex communication for its poor BER and SER. The modeling methods and conclusions are applicable to other digital pulse modulations.