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22 INTRODUCTION TO TELECOMMUNICATIONSFigure 1.4-7. First- and second-order FDM multiplexes. (a), Two-wire bidirectional circuits;(b), four-wire circuits; (c), multiplexed four-wire circuits (12 channels); (d), multiplexed four-wirecircuits (60 channels).The second-order FDM multiplex combines five group circuits (c) into one fourwiresupergroup circuit (d) with 60 channels. The bandwidth of a supergroup circuitis 5 48 kHz ¼ 240 kHz in each direction (312–552 kHz).Continuing in this way, ten supergroups can be combined into a 600 channelmastergroup, and six mastergroups form a 3600 channel jumbogroup [5].1.4.6 FDM Transmission SystemsGroup circuits can be carried on two amplified wire-pairs, with amplifiers at regularspacings. Signals of higher order multiplexes are carried on transmission systems ofseveral types.Cable transmission systems consist of two coaxial cables, with amplifiers atregular spacings. Cable systems are used on overland and underwater (transatlantic,transpacific) routes.Microwave radio systems consist of a pair of unidirectional radiofrequency (RF)transmission links in the microwave region (2, 4, 6, 11, or 18 GHz). The outputsignal of a FDM multiplexer modulates the frequency of the microwave carrier.The RF signal travels in a narrow beam from the transmitting antenna to the receivingantenna.In terrestrial systems, the microwave links are divided into several sections(hops) of, say, 20–40 miles. The transmission in each section is on a line-of-sightpath. Repeater stations at the section boundaries amplify the RF signal receivedfrom one section and retransmit it to the next section. The repeater stations are