A TDMA/PCM Experiment on Applications Technology Satellites

In recent years there has been considerable interest in time-division multiple access (TDMA) for communications via satellites. TDMA is free of intermodulation effects, since only one station signal is present in the satellite at a given time. The principal problem which had to be solved for TDMA was that of establishing and maintaining time synchronization between the participating earth stations in the communications network. In a joint communication experiment a new TDMA/pulse-code modulation (TDMA/PCM) system was built in Japan and installed at the Applications Technology Satellites (ATS) stations. The features of the system are 1) that time synchronization is established with a continuous low-level pseudonoise (PN) sequence so that the interference to other stations is negligible and knowledge of satellite range is not required and 2) that the participating earth stations have clock control loops which maintain bit and frame coherency referred to the satellite, that is, the bits as they arrive at the satellite are coherent from one station burst to the next station burst. Some of the results of extensive communication tests through the ATS are discussed.