Comparing high order pulse position modulation to burst differential phase shift keying in intersatellite optical communications links

Abstract

Distributed satellite mesh networks utilizing low-cost small satellites require communications components that have low impact on the cost, size, weight and power (SWaP-c) while long range, high bandwidth communications can reduce the number of required satellites in the mesh network. Free space laser communications provide a potential for low-SWaP, long-range communications links by leveraging high aperture gains due to short wavelengths yielding narrow divergence. Additionally, wide band optical booster amplifiers frequently operate with an average power limit rather than a peak power limit enabling low-duty cycle formats to take advantage of high peak powers. Full realization of these benefits depends on the format being utilized. New-space laser communications terminals leverage much of the existing fiber optical telecommunication infrastructure to repurpose products for long range free space applications that currently only have a launch amplifier and, potentially, a preamplified receiver. While long haul fiber applications favor binary phase shift keying formats, low cost applications of optical fiber telecommunications links that require low-SWaP on the transmitter and receiver ends of the link frequently drive designs towards intensity modulated direct drive (IM-DD) links. We investigate extending the range of a free space optical link through use of three different variable data rate methods including, reducing receiver bandwidth, utilizing burst waveforms, and pulse position modulation formats (PPM). Our results indicate that although a higher SNR is required for PPM formats, orders higher than 64 can acquire links at comparable average power and data rate as differential phase shift keying formats under similar receiver conditions.