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'.6 VSAT EARTH STATION ENGINEERING 369

introduction of digital receivers and the need to devel'op cheap, mass-prcdr:ced VSATterminals.

The VSAT earth station can now be considered as two basic components: an butdoorunit (ODU) and an indoor unit (IDU). This is illustrated in Figure i"il

i'he ODU and IDU units are broken down further in Figure 9.24, Figrtrc -.2r+ givesa typical configuration of a VSAT that has the modulator/demodulator (modem) equipinentlocated in the IDU. In some cases, the low noise block (LNB) or low noise converter (LNC)and the I{PB (high-power block) or HPC (high-power converter) lrouse the complete RF'output and Rli input stages of the transmitter and receiver, the up-corrverters and down-convefters and, 'n rr ar^'t cases, the modem. With the mass production of VSAIs, the, [-NBsand l,NCs are being developed .n application specific integrated circuits (ASICs), yeryoften grown as a single microwave rnor-clithic integrard circuit (MMIC).

The hub station has a more complex desigrr th:r rhe VSAT station sinee it not onlyhas'to handle al! of the inbound and outbound traffrc, ^ 'ather than just "thin loute" traffic of'. ringle or a few users, but it also has to tnanage the network cqnt.il functions. Figure 9.25gives the general layout of the hub earth station"

OutboundTDM channels

Hubantenna

lnboundMF.TDMAchannels

F|GURE 9.25 Schematic of a typical hr-rb master control stat ion (after 'Figure 4.2.1 of refer-ence 4. @lTU, reproduced with permission). The l ine interface equipmeht handles the terres-tr ial ports to the host computer. The control bu: via the hub control interface al lows al l 'ofthe transmit, receive, and switching functions :o be carr ied out. The transmit processing andcontrol equipment (PCE) prepares the TDM stream for the outbound l ink to the VSATs. Thisstream passes through the lF interface (the equivalent of the interfauir i ty l ink of the VSAT inFigure 9.23) to the up-converter (UC) that mixes the lF to RF. The high-power ampli f ier (HPA)ampli f ies the TDM stream and the antenna transmits the signal. On the receive side, the an-tenna passes the ind iv idua l inbound MF-TDMA s ignals to the low no ise ampl i f ie r (LNA) forampl i f ica t ion pr ior to down-convers ion (DC), demodulat ion, and so on to the user .

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