Isochronous and Anisochronous Modulation Schemes in Wireless Optical Communication Systems

Abstract

The choice of the modulation format is one of the principle factors in realizing a high performance wireless optical communication system at a reasonable cost and acceptable complexity. The purpose of this paper is to make a comparison between isochronous and anisochronous modulation scheme categories from Discrete (digital) pulse time modulations (PTM) through the simplest scheme in each family; PPM and DPIM respectively, in term of bandwidth requirement, power efficiency and transmission capacity. This is done to give a wider view on the performance of such schemas under wide range of design parameters. In this paper, the properties of PPM and DPIM have been analyzed, from this analysis it has been shown that DPIM or anisochronous modulation schemes in general are strong candidates when synchronization and transmission capacity are taken into account, and when it comes to power performance PPM or isochronous modulations are better.Pulse Time Modulation (PTM) schemes in which a range of time dependent features of a pulse carrier may be used to convey information. Discrete (digital) pulse time modulations (PTM)techniques fall into two categories, namely isochronous and anisochronous. Isochronous schemes encode data by varying the position or width of a pulse, but the overall symbol structure remains constant, in contrast, anisochronous schemes have no fixed symbol structure [3], a simple Digital Pulse Time Modulations (PTM) tree is shown in Fig. l.