Optical Intersatellite Communication Links

The performance of direct and hetedyne detection optical intersatellite communication links are evaluated and compared. It is shown that the perfonnance of optical links is very sensitive to the pointing and tracking e m at the transmitter and receiver. In the presence of random pointing and tracking errors, optimal antenna gains exist that wiU minimize the required transmitter power. In addition to limiting the antenna gains, random pointing and tracking emrs also impose a power penalty in the link budget. This power penalty is between 1.6 to 3 dB for a direct detection QPPM link, and 3 to 5 dB for a heterodyne QFSK system. For heterodyne systems, the canier phase noise presents another major factor of performance degradation that must be considered. In contrast, the loss due to synchronization error is small. The Link budgets for direct and heterodyne detection systems ye evaluated. It is shown that, for systems with large pointing and tracking errors, the link budget is dominated by the spatial tracking error, and the direct detection system shows a superior performance because it is less sensitive to the spatial tracking error. On the other hand, for systems with small pointing and tracking jitters, the antenna gains are in general limited by the launch cost. and suboptimal antenna gains are often used in practice. In which case, the heterodyne system has a slightly higher power margin because of the higher receiver sensitivity.