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.

Free-Space Optics (FSO) is a wireless optical technology that enables optical transmission of data, voice and video communications through the air, up to 10 Gbps of data, based on the use of the free space (the atmosphere) as transmission medium and low power lasers as light sources.
Quality and performance of FSO links are generally affected by link distance and weather conditions like environmental temperature and light, sun, fog, snow, smoke, haze and rain. In this paper we study the effects of weather conditions on the performance of FSO links, taking the climate of Algeria as an example, and since there is no known analysis on the effects of weather conditions in this country, this paper offers an attempt to analyze and identify the challenges related to the deployment of FSO links under Algeria's weather. We also present a Graphic User Interface "GUI" to provide an approximate availability estimate of an atmospheric optical link in term of probability of connection.

The PPM (Pulse Position Modulation) is commonly used in Free Space Optic (FSO) systems owing to its power efficiency, but it shows a rapid decline in spectral efficiency with increase in the power efficiency and moderate data rate. In order to improve these two parameters, we present a modified modulation scheme of the existing PPM, on the basis of PPM, PAM (Pulse Amplitude Modulation) and PWM (Pulse Width Modulation). This modified version called DAWPPM (Dual Amplitude-Width PPM).
The average power requirements, bandwidth efficiency and normalized data rate are studied after introducing symbol structure. The proposed scheme shows an improvement in terms of data rate and bandwidth efficiency, and when in come to power efficiency it shows lower efficiency compared to PPM. We present theoretical expressions of data rate, spectral efficiency, and normalized power requirements, and we present comparison results to PPM modulation scheme.