The purpose of this study was to investigate future GEO constellation communication scenarios and derive the resulting requirements for optical intersatellite links. Starting point for the investigations was the existing Laser Communication Terminal (LCT) developed by TESAT with the support of DLR and ESA. The existing LCT has the capability to transmit user data at a data rate of 1,8 Gbps between LEO and GEO SCs. For future GEO applications, additional features for the LCT were identified in the study. These are the increased user data rate for the return direction, a low data rate forward channel capability, and the capability to support GEO to GEO intersatellite links at distances of up to 72000km.