This paper analyzes and enhances the performance of moment generating function techniques, notably the Chernoff bound (CB) and modified Chernoff bound (MCB), is used to improve the bit-error-rate (BER) performance of an optically pre-amplified for the wavelength division multiplexing (WDM) based on the passive optical network (PON) free-space optical (FSO) communications in the presence of both atmospheric turbulence (AT), amplified spontaneous emission (ASE) noise, and interchannel crosstalk. In the absence of AT and ASE at a data rate of 2.5 Gbps on the 1550 nm wavelength, digital pulse-position modulation (DPPM) systems with coding level (M) of 2 provide about 2.9 dB improvement in average power over at a BER of (depending on the turbulence level) compared with an equivalent on-off keying (OOK) non-return-to-zero (NRZ) in the WDM-PON/FSO system while maintaining minimum bandwidth expansion to leverage the extended reach and enhanced user capacity and considered as a good solution to the bandwidth requirement for future access networks, with potential for higher data rate, improved data security. The receiver sensitivities of M-ary DPPM about 51.4 dBm (~21.9 photons/bit) (CB), and 51.5 dBm (21.4 photons/bit, MCB) can be achieved, which implies an improvement when compared with an OOK-NRZ system (~38 photons/bit) in the non-turbulent atmospheric condition. M-ary DPPM retains its sensitivity improvement over OOK even in the existence of crosstalk while predicting a lower power penalty of about 0.2–3.0 dB for weak turbulence at low coding level (M) 2 in WDM systems.