TABLE OF CONTENTS The IF and baseband equipment, illustrated in Figure 9.1, establish the basic service capabilities of the Earth station, including modulation and FEC, multiple access method, and interface to the user or terrestrial network. In many cases, this portion represents the largest investment on the ground, particularly for a major gateway or network control facility. That is because, while the RF terminal usually is fixed for a particular application, the quantity and the design of the baseband equipment must be adjusted to support the bandwidth and communication applications the stations serve. This section reviews typical configurations of the major elements of this section of the Earth station.
The most essential portion of the baseband-to-IF chain consists of the modulator and demodulator or a combination of the two, called a modem. On the receive side, the demodulator detects the incoming carrier, synchronizes the data, performs error correction and outputs a clean bitstream for the particular application. The transmit side works in the opposite direction. Threshold performance in terms of error rate and synchronization is determined at this level of the station. Because there must be one modem for each frequency, multicarrier FDMA stations can contain large quantities of these units, representing a sizable investment. In TDMA star networks, some frequency channels are shared by several Earth stations. There would need to be only a single modem per Earth station, since it can receive the bursts from the other Earth stations so...
