TABLE OF CONTENTS Digital transmission relates to the link for which the MES terminal is designed to produce the digital signals by PC or modem and to send them through a transceiver. Moreover, it is possible to transmit analog signals (voice or broadcast) in digital form. Although this choice implies an increased baseband, it permits signals from diverse origins to be transmitted on the same satellite channels and the satellite link to be incorporated in ISDN, which implies the use of TDM. The digitization of analog signals implies the stages of sampling, quantization and source coding. The simple digital transmission chain includes a transmitting and receiving segment. However, the first unit of the transmitting segment is TDM with input signals from digital (direct) and analog (via encoder) sources and after multiplexed signals pass devices such as: Data Encryption, Channel Encoding, Scrambling and Digital Modulation, where the digital signal is transmitted through up or down satellite links. On the other hand, in the receiving segment of reverse mode incoming signal goes through devices such as: Demodulator, Descrambling, Channel Decoding, Data Decryption and TDM Demultiplexing in its direction to different users.
Digital signals use the same principle to modulate a carrier as analog signals. The AM, FM and PM schemes are all applicable to digital modulation; digital equivalents are ASK, FSK and PSK and special hybrid modulation solutions have also been developed to optimize digital modulation, such as QAM. The pioneering generation of MSS of Marisat and the first Inmarsat-A SES used...
