Adaptive subcarrier intensity modulation for free space optical communication

Abstract

Increased capacity demands and radio frequency (RF) congestion impacts on current communication networks have brought greater attention to free-space optical (FSO) communication as a viable augmentation technology for terrestrial, aerial, and space-based communication infrastructure. As a complementary alternative to RF communication systems, FSO can support high link bandwidths and provide high data security without RF spectral constraints. The performance of FSO links, however, can be significantly impacted by receive power variation caused by propagation and scattering losses along with losses due to atmospheric turbulence. Depending on the FSO application, these loss mechanisms can dynamically change, impacting link performance at different time scales. We investigate subcarrier phase-shift keying (PSK) and quadrature amplitude modulation (QAM) intensity modulation that can be adapted to dynamically changing link conditions to optimize bandwidth utilization. Using custom subcarrier intensity modulation (SIM) modems, the performance of binary PSK (BPSK), QPSK, 8PSK, 16APSK, and 16-QAM waveforms is reported. The impact of adaptive equalization is also characterized, and the initial performance of a subcarrier multiplexed system is presented. This work represents the first experimental evaluation of SIM waveforms using a laboratory scintillation playback system based on scintillation recorded over real-world propagation paths.