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1.5 DIGITAL TRANSMISSION 23land based, and terrestrial microwave systems can therefore be used on overlandroutes only.Satellite microwave systems use communication satellites. These satellites arein geosynchronous orbits, some 22,300 miles above the equator. At this altitude, asatellite circles the earth at an angular velocity equal to the angular velocity ofthe earth’s rotation. When observed from a point on earth, the satellite thereforeappears in a fixed position, and only small and infrequent adjustments are neededto keep an antenna of a ground station aimed at the satellite. Each satellite link consistsof an “uplink” from a ground station to the satellite, and a “downlink” from thesatellite to another ground station. The satellite amplifies the signal received on the“uplink” and retransmits it on the “downlink.”Satellite microwave systems were introduced in the 1960s and have been usedextensively on transoceanic routes. A disadvantage of satellite transmission is thelong signal propagation time (in excess of 250 ms from ground station to groundstation), which can cause problems in telephone conversations.1.5 DIGITAL TRANSMISSIONAnalog signals (a) can be converted to digital bit streams (d) and transmitted ondigital transmission systems (Fig. 1.5-1).Digital transmission is more robust and of better quality than analog transmission.While digital signals require more bandwidth, digital transmissionsystems are often less expensive than their analog counterparts, and digital transmissionis rapidly replacing analog transmission in telecommunications.1.5.1 Pulse Code ModulationPulse code modulation (PCM) is the earliest method for analog-to-digital (A/D)and digital-to-analog (D/A) conversions used in telecommunication networks[4,5]. The basic idea dates back to 1938 [7]. However, PCM requires a fairamount of digital signal processing and only became technically feasible after theFigure 1.5-1. Digital transmission system. (a), Analog signals; (d), digital bit stream; A/D,analog-to-digital converter; D/A, digital-to-analog converter.