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Chapter 18 Blake 1

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Chapter 18 Blake 1

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claudinejene.reyes25
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J81 Introduction 182. Terminal and Repeater siting 18.3 Path Calculations: 84 Fixed Microwave Links 18.3 Local Microwave Distribution Systems: Terrestrial Microwave Cc-mmunication Systems ._— OBJECTIVES After completing this chapter, you should be able to’ Describe the basic structure and uses of microwave radio links Explain the methods used in choosing sites for repeaters, * Calculate the signal strength at the receiver for a variety of transmitter antenna, and terrain configurations, * Calculate the required clearance of a microwave path from an obstacle + Calculate the noise temperature and carrier-to-noise level of a mi crowave system, + Calculate the energy per bit per noise density ratio for a microwave system, + Explain fading and describe the diversity schemes that are used to overcome it, « Describe the transmitting and receiving equipment used for FM, SSB. and digital systems, be the types of repeaters used for analog and digital systems and + Deseril jons for these systems, perform frequency cale\ Describe the place of microwave systems for terresti! broadcasting ations i Non, communtos tor 100P A gw York and Boston. fy eed Mayr repeaters 10 COVE thy aushave many uses. THEY muliath propaga fang dv mene waveleng oxy ad fe ese ipa 1010 dv ec aunt fading on a moment-to-moment basis, frequency diveiy fo pret eters and two receivers, separated in frequency. Ideally the ‘ram que often mits the separation to 2%. Providing dual transmitters andre ‘znen is expense, bt it als allows hor standby protection—if one transmiter ce receiver should fail, communication can continue uninterrupted. Figure 18 5i- Iustrates frequency diversity. Receiving = Antenna f \ | Reooiver 5 onion Pa =» Baseband oat Transmitting | ’ Aenna i Reoeivor 2 wt Ce andy prtectiog fa nmetimes ting tum % pro equi toca gency ets AF Sch that an extra channel ae fenna. Ths jg length by on Y: Another way to prevent multi” iy Called VNR either the transmitting or the" ee ww Hace diversity, generally involves ps t i } | \ i Figure 18.4 Terrest tems a tems u: amplit ture an tum. Section 16.4 © Flued Microwave Links fas; sanas one above the other on the same tower ted by 200 wavelengths or more. At 6 Gi uted. Since the lower ofthe two antennas must be high enough fa igh enough for reliable line «org communication, space diversity we enna. Space diversity is illustrated in Figure 18.q we &® Well as more iy eto antennas should be sepa 1, & separation of 10 meters is te Alternate Receiving Antenna Propagation Paths paseband in -—Pabnisrditney | i — Receive Baseband out 7 t ‘Transmitting . Antenna Alternate Receiving Antenna Figure 18.6 Space diversity isthe advantage of a hot standby system clink? Wha we of a hot standby system in a microwave link SECTION 18.3 REVIEW = QUESTION, 184 Fixed Microwave Links Terrestrial microwave links can be either analog or digital systems. Most new sys tems are digital, but there are still many analog installations in use. Analog sys tems use either frequency modulation (FM) or single-sideband suppressed-carrier amplitude modulation (SSB). Digital systems use phase-shift keying or quadra ture amplitude modulation (QAM). We shall examine each of these methods in tum, FM Systems The transmitter and receiver topologies shown in Figure 18.7 closely resemble those in use at lower frequencies. Typically the IF is 70 MHz and narrowband FM is used, go that the transmitted bandwidth is only slightly more than twice that of | shown at the transmitter input and re~ the baseband signal. The baseband signal cever output can consist ofa single broudcast- quality television signal or «pum: er: veomale combined using frequency-division modulation. Typical tand, from 600 to 1800 voice channels (one to three mastergroups) can De carried Per FM carrier. The transmitter block diagram in Pig FM signal is generated at a relatively low freq ‘wired transmit carrier frequency. The local ov scillator such as a reflex Klystron or a Gunn device, eillator followed by a frequency multiplier Us Benerally under 10W, with 1-2W being typical. A nag ommonly used in the ourput stage though for Po re 18.7(a) is quite conventional. The uuency, then mixed up to the re- ator shown can be a microwave ‘e, or it can be a VHF transistor actors, Output powers are Ting-wave tube (TWT) is s near the low end of amcor 6 Eragon 1 torrente Ante corn votiy — aer xe oe ¥ uid agyplifer te oo Miceowean Preemphasie Onl Muesli Baseband y Output Miser aa De-emphs “ FM ir rector Amplifier Local Oscillator eld-effect transistors ¢ er using fi ate power amplifier Using the above range, a solidst : block diagram in Figure 18.7(b) show's an RF amplifier by The receiver block diagra is not always presen Tow-noise mixer (often using a Schottky diode) tha nt, particularly in older designs. Unti the mixer, but this rly it was easier to build a A amplifier. Low-noise amplifiers involved exotic techn 2 low-noise microwave say such as parametric amplifiers and masers and were not economical for everyday use. However, most recent microwave receiver designs use (ats FET transistors ina low-noise amplifier (LNA) before the mixer The intermediate frequency is generally 70 MHz, but the IF bandwidth d pends on the bandwidth of the signal, which varies with the baseband sign! bandwidth Asi ¢ installation can use more than one carrier, depending on the requ! trafic capacity and the available spectrum sp: could use 12 for sidebands, We EXAMPLE 11 genes anaes receiver in Figure 18,7(b), the received ca Mency ifthe receivg te IF is 70 MHz. Calculate the local owiltor® SEWET Uses low-side injection ace. For instance, a 12 GHz sss carriers, with a 20 MHz separation between carriers to allow Solution This is Oscillator frequen for, the local on 4 standard su fron al oscil the epethetetodyne receiver, Subtract th incoming f i frequency should give the tor frequeney quency should ho = 6879 MHz oy ea Section 18.4 © Fixed Microwave Links — 687 pital data can also be transmitted with FM i sic a systems, us ternal en ee alae ee ee tasted over : s emote ficient (0 se digital iia are necessary ii ‘ic : gh diastcs A yin any taicrowave system that extends for more thin te aneously trans eater receives from one direction on one fi ates a ee trequeey. Dif aa for transmission and recepti ss ress i =ption t¢ id GiStiommrsrnsoetnieemnn ee oe TE cer ea bidirectional, so they transmit and receive in twp prevent feedback, but bo receive frequencies at a relay site must be differ- uw receivers. Adjacent Leen ee . sin fequencies as illustrated in Figure 18 ee Receive | Antenna | ‘Transmit! Antenna Station A Figure 18.8 Bidirectional system ‘Two basic types of repeaters are used in analog systems. Both are shown in Figure 18.9, In Figure 18.9(a), the repeater moves the received signal, by mixing, ‘oan intermediate frequency (IF) to amplify and filter it and then moves it to the new transmitting frequency, all without demodulating it. The frequency of the shit oscillator is equal tothe difference between the input ‘and output frequencies ofthe repeater, that is, foo = fi ~ Jol (18.11) Where fi = shift oscillator frequency of, = input (receive) frequency f= output (transmit) frequency so that the sum of the I al to the output carrier frequency, The local oscillator frequency is set local oscillator frequency that is, “4 the intermediate frequency is ¢q¥ Repeater Station B ‘Transmit Antenna h Receive Antenna Presents Power rower "Kates +( ‘Transmit ‘Antenna Therefore, } fio = fo ~ T0MHz (a8) ' “The shift oscillator frequency is added to or subtracted from the local oie in order to lower or raise, respectively, the signal frequeny hrough the repeater. An example should make this clea. frequency in mixer # as the signal moves t Ra EXAMPLE 18.10 A microwave repeater has the block diagram shown in Figure 18.9(a). The received signal has a carrier frequency of 6870 MHz, nd the transmitted signal is to have a carrier frequency of 6710 MHz. The IFis 7 MHz. What should be the frequencies of the local oscillator and the shift oscillator? To what frequency should the output of mixer #3 be tuned? Verify these results by following the signal through the repeater. f Solution The shift oscillator should be set to the difference between te transmit and receive frequencies: i fso = 6870 MHz — 6710 MHz = 160 MHz The local oscillator frequency should be: ; fio = f. ~ 10 MHz. 6710 MHz — 70 MHz 6640 MHz Mixer #3 will produce an output equal to the sum of the (wo ocr quencies, that is, \ 6640 MHz + 160 MHz = 6800 MHz Section 16.4 © Fund Microwave Links 6am sxer #1 combines the incoming frequ . Mie io get the TF of 70 Mite? 9870 MHz andthe otpu of 6870 MHz ~ 6800 MHz = 70 MHz ris fequency i aed 10 the local osilator frequencies o get So = 70 MHz + 6640 MHz = 6710 MHz. ‘The same oscillator frequencies co tp change a received frequency of 67 6810 MHz. The local oscillator would h; tld be used to go the other way, that is, 10 MHz to a transmitted frequency of lave to be retuned to a frequency of 6870 MHz ~ 70 MHz = 6800 MHz and the output of mixer #3 would have to be tuned to the di two oscillator Frequencies, which is wm the difference between tho 6800 ~ 160 MHz = 6640 MHz When this frequency is subtracted from the incoming frequency in mixer #1, we stan IF of 6710 MHz — 6640 MHz = 70 MHz When it is necessary to connect to the baseband signal, for instance, to add or «top groups of telephone signals, a baseband repeater like the one shown in Fig- ute 18.9(b) is required. In a baseband repeater, the signal is demodulated, then re- ‘ansmitted. This type of repeater is just a receiver with its output connected to the ‘odulation input of a transmitter. If baseband access is not required, IF repeaters add less noise to the signal ttan do baseband repeaters. The noise performance of repeaters is very important inanalog systems, because noise is cumulative throughout the chain. Once added, ‘wise cannot be removed from an analog system. Single-Sideband Systems Some recent analog microwave systems use SSB in order to reduce the bandwidth "quired, The reader should recall from basic communications theory i tte ‘undwidth required for SSB transmission 1s equal to the baseband ba . ‘ile the bandwidth needed for FM is always more than wice as Feat va bandwidth. This bandwidth-conserving P' ea bce For instance, ‘onsderable increase in traffic-handling capacity along MOY rey. Oe "M microwave links inthe 6 GHz band {Pica eae an support 6000 ‘tamels in bandwidth of 29.65 MH. Pre same "ice channels (ten mastergroups) USINE Pe are already multiplexed Is imeresting to note that ansl0B Vole on pe rans a web ens FM or panne ‘hen the FDM signals were creat ‘th efficiencies establi "fora aang ‘of FDM gen that used fr FM, except that the TF S85 crow i wnt Frese! and care must be taken foneney ‘ily 74 Mae instead of 70 MHz and srest is usually 74.13 | tiade-rorlation any amp! ea Like ofthe baseband signa ec ear eng nou sala ar 2135 Made modulation wih igh pot ponies OF 5 nS eye | ' aevantages in microwaVe radio Links Fig. signa jee This requites demodulating th ch repel modulating OM a NEW carrier. As ding se link is high enough to avoid emo, tee oe ie prees tough te wil eno increase Ineo" HO ainable, but the error rates will add fro pric. 2.7270 plying asin analog systems. 1 ik a ae ee employing digital modulation techniques i The second mat compatibility with digitally-coded baseband tomate division multiplexing can pass through digi ee he data. the . yen recodin the sig Digital signals vsit crowave systems without alteration. ‘The only disadvantage of digital transmission of voice and video signals that it equires more bandwidth than analog FM or SSB. However, as better pression algorithms are devised, this is becoming less of a problem, ory In practice, most systems still use 64 kb/s for each voice channel. ‘To be transmitted by microwaves, a digital signal must modulate microwave-frequency carrier. Two modulation sche! rowave radio: phase-shift Keying (PSK) and quadrature amplitude module (QAND, which involves both amplitude and phase shifts. These systems were cussed in Chapter 12—the emphasis there was on telephone modems, where ¢Ramel bandwidth is only about 3 kHz, the carrier frequency is on the er Kilohertz, and the data rate is es are used for digi i a few kilobits per second. Microwave systems much higher carrier frequencies, bandwidths, and dat 1 g principles are the same = more recent and the more sophisticated the system dae ed Recall that a symbol is transmitted whenever the = sat changes can involve amplitude, paseo 82 i ible but are not used in modern microwave " formation fone! mation transmitted with each symbol is # fi" States: In general bits pe symbol miter changes state State: th WeNCY shifts are also umber of bits of ‘he numberof possible ot Na = log, Ns bits Sum Per symbol nbe of possible states Per symbol bau eis the number of symbols per second wlio Most digital radio systems use judes and four phase angles fora total of folethe second has eight amplitudes and ei gersymbol. The current state of the art i¢ «ymbol. Experimental use has been made oi the disadvantage, common to all AM o ransmiltet. FSK and PSK schemes denon canbe more efficient. (See Chapter 2 for plies.) On the other hand, QAM has gre a given transmitter power and data rate, a ™s, of requiring linear amplifiers in the Fequire this linearity, so the transmitters @ discussion of linear and nonlinear am- ater noise immunity than FSK or PSK for ind it requires less bandwidth. i EXAMPLE 18.11 A typical microwave digital radio system uses 16.QAM. Ithas a bit rate of 90.524 Mb/s (two DS-3 signals or 1344 voice channels. plus overhead—see Chapter 8 for a discussion of digital signals in teleph ony). Calculate: (@) the number of bits per symbol (b) the baud rate Solution This problem should remind you of some we did in Chapter 12; only the numbers have changed (@ There are 16 possibilities, so each symbol can transmit a total of logs 16 = 4 bits. (6) Therefore the baud rate is one-quarter of the bit rate or 22.631 Mbaud. Aside from the modulator, transmitters and receivers for digital microwave analog systems. Repeaters, however, must demodulate u es of digital transmission. That is, tadio resemble those for the signal to baseband to achieve the advantag they must be regenerative repeaters, as shown in Figure 18.10. a Data aa : _ nae = Transmit Antenna Receive Antenna Figure 13.10 Regenerative repeater has been distorted or comuped by 4 Sock repeaters can restore San readable atthe ever signal ise to its original state, as long as the sie! : : omamuni TION 18.4 REVIEW have when used for digital SECTION » What advantages do regenerative repeaters ystems sy use terrestrial microwave yp co 1 naetemet access, ANd telephony a Mocal multipoint Communic f GHz range. There is also ayant Print distribution system) whch beginning with MMDS because js call cies in systems have Been OPETANE fot seve sts were analog, BUt TECENt VErsOns jp a vr cable television using coaxial cab. 8 a ar comadictOTY name wireless cable for thy ut of atypical MMDS. Local television stato tind, which alSO PiCks up cable-network station ‘digital format called MPEG (for Mo. rows the Tay‘ head rebroadcast in we oie The signals a ee site. Te yp) BY iCTOWA FON OO tall towers to smal fo Pace Ee7s Band apartment buildings. The system works oer wall-mounted ant srs dss 150 km, depending on the antenna heights, Re. dn nmennas must have a direct Fine of sight to the transmitter, so the systemis vis or dense metropolitan areas with many tall buildings tneconomical in hilly sean te sgn. Tere i suflicent spectrum for about 100 chan, up to about ‘Microwave Downlink for ‘Television and Data Signals @ RF/Telephone Modem = |* Data Uplink via lobe lephone Line MMDS syste MDs, Subscriber 8 boy ° TS are Used fi 3 Oly those packer emitted or Imermet access. Typically, all data request 1S that are ag O"® RF channel, and each home ae sed to it. In this way the system es , . syn be expensive. With an existing Mi {pred for uploading data tothe internet or for ebro must use some other technology. Usually macnn ee tevison r zy. Usually upstream communication is secomplished using the PSTN, but subscribers find this inconvenient, especially 88 it ties up a forinternet access, as it ties up a telephone line. Recently some MMDS operators juve begun t0 investigate the possibility of using a low-speed wieless link, such Z rs fo pares oor tation. The only problem with this is that it adds pai fe system, which must rem: itive with ca television in order to survive. ag ee ie oe Most observers expect that MMDS services will eventually be replaced by IMDS, which we look at next. Another possibility, since many MMDS services we owned by telephone companies, is that these services will be replaced by video services delivered by telephone line, when these services become available. LMDS: ‘The concept behind an LMDS is similar to that behind the earlier MMDS, but with some major changes, First, moving the frequency to 28 GHz allows much freer use of spectrum; about 1.3 GHz is allocated to this service in the United Suates, and 1 GHz in Canada, Second, the high frequency causes severe attenua- tion problems in the presence of rain, and even foliage from trees can block the signal. This, coupled with the difficulty of generating substantial amounts of power at 28 GHz, forces the system to use much smaller cells with a maximum diameter on the order of 5 km. Finally, the LMDS is designed to be bidirectional This allows it to be used for internet access without using the PSTN and, in fact, allows the LMDS to be used for telephone communication instead of the PSTN. See Figure 18.12, Video Downlink Full-Duplex Data Link Pimure wa.s@ MDS system mm f tranciver | 1 11S in New York ¢ ayo as SP (AMPS. 0201 ytrom eas a“ ioe ot ‘ sonnet? ms pvailable, IMDS of-sight technology Howe pst ws 1M isi rort wavelength, LMS may coverage in some ares seoveh so : ysmitter “oo om a tran une oth fr. | | vaste io cea Reusable Building blocks direct signal ‘igure 18.13 Malia propagation and LMDS systems SECTION 18.5 REVIEW OUESTION Beefy expan he diferene between LMDS and MMDS systems, fom he view ofthe user rious research FONANES, One of the dO" ans to transit power mae rr acca he ‘ranemitted could to fd then add the path of the directory with the If the file is located in the “c:\myw {ype the following at the MATLAB prompt ‘Transmitter carrier frequency in Megahertz = 6000, System bitrate in bits/sec Transmitter power in watt ‘Transmitter antenna gan in dBi Receiver antenna gain in dBi = 25 Minimum distance bet km = 20 Maximum distance bet km = 60 ———— ron this chapter: ats to rememt -range communication sy acs co ooslel 100 alas nen rial microwave links a Tangerang Systems SPanniN (Pouvan ee Here | suMMARY 1, Terrest Fi and, when repeaters are use, kilometers. ropagation and require repeater, eotsight prop i mc aes use Incase EAN 2 Miraely every 40 km, depending 9 tm well above cba selva eb to Soop cbecorevs Oe eo | 3 Sand wo avid signa oss doe IMEI ive a ais ' signe gin stb chosen 1 868 iy 4, Transmitter power ane for analog systems. 1n digital systems, the equivajey rier power-to-noise specification is the enerBY 5, Fading is common with mi frequency or space diversity: 6. Analog microwave systems use either F tuse PSK or QAM 7. Microwave systems can be uses Intemet service to consumers. directional broadcast systems. se density ratio, per bit per noi rowave signals. Itcan Be OVEFCOME bY Wing ei, M or SSB modulation. Digital system, 1d for distribution of television broadcasting ay, ‘Two-way systems are beginning to replace un, ee neerennens EA \ mporrant EQUATIONS \ 5 (L = 1)290 + Tay d= Vith; + Vie gt) 7, zt (sn) ; (18.2) Toy = 290(NF ~ 1) «iss on (189) (18.3) te No = kT ig Pe °F (dB) = Gy (dBi) + Ge (ABI) Ny = logs Ny (0) = (244 + 2logd'+ Wlogf) (18.5) Py = KIB (18.6) GLOSSARY seeeweneeeeen mon carrier-to-noise ratio The signal-to-noise ratio ina receiver at resmel zone A region near an object in which dao © 2 point before the detector. fects are significant. | Mitre The deviation os wave asi pases an obstacle or op A single transmission pa rom wns passes through a small aperture. Jitter Abrupt variations in the timing of » digital 8° ‘diversity Use of more than one frequency or transmission path, ‘o improve system reliability inthe presence of fading. energy per bit Energy received in the time taken to transmit ‘ne bit. _ fading Variation in received field strength over time due to ‘changes in propagation conditions. local multipoint distribution system (LMDS) Ns microwaves for two-way transmission of telePi and high-speed data ‘multichannel multipoint distribution Lorin restrial microwave system for the distriboi0® net, and telephone services to businesses and “a receiver-transmitter combination that amplifies and eis signal. gestions rely compare microwave and fiber-optic links, in terms of st, and need for rightof-way ‘what are repeaters? Why are they needed in long-haul ter- Weld microwave systems? 4, approximately how does the line-of-sight distance for ra- ‘dowave propagation differ from that for visible light? _.wnatgpe of feedline is typically used for microwave ys- wer why? 5, Describe 4/3 earth graph paper and explain its use. {Whats a Freel zope? What is its importance in mi Sowave communication systems? 1 whys it advisable ina terrestrial nierowave system to keep petmena gain under about 40d? 2 8, Why is the term carrier-to-noise level, rather than signal-to- noise level, used for path calculations with FM microwave systems? 9, What is the equivalent to carrier-to-noise level for a digital microwave radio system? How is it calculated? 1. What causes fading in microwave radio systems? Il, Name two types of diversity, and describe how and why they are used. 12. Which type of diversity is preferred when spectrum space is ata premium? Why? PROBLEMS Snes Section 18.2 23. How far from the transmitter could a signal be received if the transmitting and receiving antennas were 40 mand 20m, respectively, above level terrain? %. A transmitter site is on a hill 40 m above average terrain and Uses a tower 20 m in height. How far above average terrain ‘ould the receiving antenna have to be for reliable commu- % ‘ication over a distance of 45 km? ‘Suppose there is an obstacle midway between the trans ‘er and receiver in the previous question. By how much must ‘he path between the towers clear the obstacle in order to mi ‘oid diffraction at a frequency of 11 GHz? be ‘Many repeaters would be required in & system span- "ing 2000 km if the towers are on average 40 km apart? How many repeaters could be eliminated from the system in ‘Se previous question if the repeater spacing could be in- ‘eased to 45 km? est a Se RT TET map A map showing surface feature, inciting the elevation of the terrain. 13, What is a hot standby systeen? Why is ita desirable feature of a communication system? 14, What range of power output level ritter in a point-to-point microwa\ 15, Why did microwave receivers lack an until recently? 16. How can digital data be communicat crowave systems? 17. What advantage does SSB have over FM for microwave sys tems? 18, What advantages does di for microwave systems? 19. How do repeaters for digital systems differ from those used in analog systems? 20. What modulation schemes are typically used with digital microwave radio? 21. What advantages does the use of a frequency of about 28 GHz give to LMDS systems compared to earlier mi- ‘crowave distribution systems? 22. How does the data rate achieved with LMDS compare with other high-speed Internet access systems such as ADSL and cable modems? Isis typical for the trans ve link? RF amplifier stage ed using analog mi- . ital transmission have over analog Section 18.3 28, Suppose the transmitter in Problem 22 has an output of 2W te fedinc ha ax of Band the anes pa is 28 dBi, Calculate the power density a 38 iy atthe receiving an- 29, The transmitter in the previous problem works with a receiv ing ins - Bi. afeodliag ing installation having an antenna gain of 32 dBi, a fecdhine Joss of 1.5 dB, anda receiver noise figure of 25 4B. The ‘bandwidth is 20 MHz at a carrier frequency of 6 GH. (a) What isthe power at the receiver input, in dBm? (©) Calculate the antenna noise te ae : smperature, referred receiver input, assuming a sky temperature of 130 Ke (©) Calculate the receiver noise temperature. 2 (©) Calculate the noise power, referred F 10 the receiver input “aestz, how many channels occupier’

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