Spectral analysis of intensity modulation schemes in free space optical communications

In this study, power spectral densities (PSDs) of intensity modulation schemes used in free space optical communications namely, on-off keying (OOK), pulse-position modulation (PPM), digital pulse interval modulation (DPIM), pulse amplitude and position modulation (PAPM) and differential amplitude pulse interval modulation (DAPIM) have been derived. The bandwidth requirements of these schemes are determined from the occurrence of the first null in the PSDs. For comparison we have considered OOK, 8-PPM, 8-DPIM, 2 × 4-PAPM and 2 × 4-DAPIM. Among all the schemes considered, the bandwidth requirement of 2 × 4-DAPIM is the least. Although 8-PPM has the least effect of baseline wander and the maximum detected optical power among all the schemes considered, the bandwidth requirement is very high. This may not be desirable in certain cases. The PSD curves are quite important in the link design as these will give information on how the signal power is changing with the bandwidth. If the noise PSD is also known then these curves can be used to determine the optimum bandwidth for maximum signal-to-noise ratio. The results are helpful in the selection of modulation scheme for free space optical links and to determine how the power content of the signals is affected by filters and other devices in the link.