Click for next page ( 24


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 23
WORKING PAPER SECTION 3 THE SPACE SEGMENT (TRANSMITTER)-SURFACE SEGMENT ( RECEIVER ) P OWE R BU DGET FOR A UH F DB S-A SY STE M- SE RV I C E This section of the paper addresses the following question: What basic space segment engineering characteristics, especially RF and DC power and bandwidth, are required to provide the surface segment--the receivers of the listening audiences--with the availability, quality, act reliability of service necessary to capture and keep their attention.?~ It should be appreciated that while the technological and operational characteristics suggested here are rational, reasonably well informed, and adequate for an initial paper addressed primarily to other communications engineers, these characteristics are neither comprehensive nor described in the detail required for space and communications system engineering. Correcting errors of facts and/or judgement and "putting flesh on the bones" must be the challenge to, and the responsibility of, others. Individual Circuit Reliability A fundamental assumption made here is that given the inherent dependability of UHF line-of-sight radiowave propagation along higher angle, unobstructed space-to-surface paths, the system engineering parameters should be chosen to see circuit reliability at the 99.9 percent performance level. This reliability should be limited only by any failure of the space segment or the surface equipments to provide the operating performance they were designed to exhibit. That they do so is a matter for communications and space engineers to ensure. Of course, the experts must balance performance--quality, ease, and flexibility of operation--against the cost of production, distribution, and use of both the space and surface segments of the system. With some important exceptions, the circuit characteristics are all essentially constants. With the space signal arrival angles considered here, i.e., from the subsatellite point (9O) to the Arctic and Antarctic circle regions (15-20), there will be no fading of the kind experienced on surface-to-surface, line-of-sight circuits, where the arrival angle is usually well less than 1. The exceptions, which must be dealt with statistically and about which present statistical data are inadequate, i ncl ude the fo] ~ owl ng: 14. The provision of a broadcasting service that would meet sound engineering and operating standards is a necessary but not a sufficient condition of successfully gaining and retaining audience attention. The program content will determine "bottom line" broadcasting service success or failure. - 23 - WORKI NO PAP ER

OCR for page 23
WORKING PAPER l. The influence of structures that might intervene in the space transmitter-surface receiver radiowave propagation path. 2. The influence of foliage that might intervene. 3. The effect of tropospheric rainfall conditions. 4. Attenuation caused by the total of building structures plus foliage. S. The influence of rough terrain. 6. The effect of scintillation fading. 7. Frequency selective fading. The Influence of Structures Some building wall structures will attenuate 2.5 GHz signals more than others. The attenuation exhibited can be influenced by intense incident precipitation when sheets of water can form on the structures. Some receivers will be effectively out in the open, some shielded by one wall, some by more than one. Some receivers wild only receive signals directly from the transmitter; others will receive additional (in and/or out of phase) fields reflected from one or more nearby structures. The likelihood that received signals will be appreciably attenuated by building walls is greater the farther the receiver is from the equator, and may be greater in colder months. Other nearby buildings, especially in urban areas, can "shadow" a receiver from the incoming signal. Only in situ measurements on or near the expected operational frequency, and for various signal arrival angles, over a sufficient length of time in various locations within the regions served will provide the statistical data base needed for satisfactory engineering and operational judgments. As a first, somewhat arbitrary but reasonable, estimate the following building structure attenuation circumstances are assumed: o On average, 25~ percent of all receivers at all times will experience essentially no radiowave attenuation beyond the inverse distance-squared free space path loss. O On average 25 percent will experience 3 db. 0 25 percent, 6 db. 0 l5 percent, 9 db. 0 7 percent, ~ 2 db. 0 3 percent, 15 db. - 24 - WORKING PAPER

OCR for page 23
WORKING PAPER Thus, on average 25 percent of the space segment's transmitter beams would have to radiate twice as much RF power (and therefore would require twice as much space segment DC power) as those 25 percent of the beams that would have to radiate just enough power simply to overcome the free-space radiowave path loss in providing the required quality, reliability, and capacity of service. These assumptions suggest that on a statistically weighted average basis the space segment's overall RF power output would have to be 5 db greater than that predicted from consideration of free-space propagation conditions alone. The Influence of Foliage Increased density of foliage and, in some cases, the incidence of intense precipitation on foliage, can increase radiowave path loss.~5 It is generally more likely that foliage attenuation rates experienced will be higher for receivers located nearer the equator and during the warmer months. But the actual overall attenuation could be greater in areas far removed from the equator when low-elevation angle reception would see the radiowaves traverse greater foliage path lengths. The same somewhat arbitrary but reasonable statistical distribution of foliage attenuation levels is assumed as that previously described for building structure attentuation. The Influence of Tropospheric Rainfall Even at the 99.9~+ reliability level, it does not appear necessary to consider providing more than a fraction of a db to accommodate excess signal attenuation in tropospheric rainfall conditions. Attenuation Caused by the Total of Building Structures Plus Foliage Over the large broadcast area expected to be served by any one space segment (some 7.5 million square miles; some 20 million square kilometers), climate and weather circumstances can be expected to increase structure radiowave attenuation considerably but not to increase foliage attenuation simultaneously. Thus the increased power demands required of an alert and dynamic space segment that is informed about the climate, weather, and channel-beam use circumstances throughout its service area could be designed, and could dynamically comport itself, so as not to require nearly as much increased RF power output as the 2 X 5 db = 10 db of additional structure plus foliage attenuation might suggest. It is assumed here that a total of 7 db would be required. l5. "An Analytical Study of Wave Propagation Through Foliage," G.S. Brown and W.~. Curry, RADC-TR-79-359, Rome Air Development Center AFSC, Griffis Air Force Base. N.Y. 13441, 1980. "An Initial Critical Summary of Models for Predicting the Attenuation of Radio Waves by Foliage," M.A. Weissberger, ECAC-CR-80-035, Electromagnetic Compatibility Analysis Center, Annapolis, MD 21402 (1980~. - 25 - WORKING PAPER

OCR for page 23
WORKING PAPER The Influence of Rough Terrain In addition to structure- and foliage-related excess attenuation, an additional power budget margin must accommodate those spacewave receivers that may be partially shielded from the space-radiated signals by rough terrain. In the limit, some receivers will be found to be well within the diffraction, rather than the line-of-sight, radiowave propagation regime. Because the diffraction field attenuation rate at 2.5 GHz is great, receiver locations well within this regime can lose reception altogether. Clearly, the greater the system power margin provided, the greater the fraction of total diffraction field receiving locations that can be served satisfactorily, and the greater the space segment cost. Considering all demands for space segment power, the peak DC power level will probably have to be in the kilowatt range, and this margin-cost balance is a particularly important consideration. Under many receiver location circumstances, considerable accommodation to such additional losses can be obtained through increased receiver antenna gain, at the cost of increased directivity restrictions, through increased receiving antenna heights, at the cost of reduced receiver transportability; and by accepting a lower receiver S/N, at the cost of reduced quality of reception. It is beyond the scope of this paper to attempt to relate all of these circumstances and considerations and the physical characteristics of the actual regions to be served in a quantitative statistical manner. Rather, somewhat arbitrarily, an additional margin of ~ db is provided for service to these locations. Scintillation Fading of Signals Occasionally and in some areas, ionospheric conditions can cause rapid scintillation fading of signals received after they transverse the ionosphere. The rate of occurrence and duration of important fading levels at a radiowave frequency as high as 2.5 GHz, the surface areas over which such fading would be experienced at any one time, and the likely times of occurrence suggest that the system power margins and dynamic response characteristics employed to accommodate the anticipated structure-, foliage- and rough-terrain-related increased radiowave path losses should be sufficient to accommodate to scintillation fading as well. (See the more detailed discussion of this kind of fading in Section 6~. Thus the total system margin included to accommodate, statistically, all propagation-related excess path losses is 7 db + 8 db = 15 db. This margin would be in addition to a 7 db margin for ordinary system operating misalignments and degradation, for an overall total of 22 db--a11 at the S/N = 45 db level. This total of 22 db for system operating margin may impress some satellite communications engineers as extremely high. They would be concerned, quite properly, about the additional design and cost burdens, and - 26 - WORKING PAPER

OCR for page 23
WORKING PAPER the need to confine surface flux densities to a sufficiently low level to obviate concern for meeting interference standards that providing such a margin would present to the system design engineer and the service user. But, for the most part, these engineers are accustomed to dealing with radiowave path losses between space transmitters and carefully sited surface receivers whose antennas are located external to buildings and have an unobstructed, high angle-of-arrival, radiowave view of the geostationary signal source. Of course, it may weld be that better statistics and/or a different judgement concerning the acceptable trade-off between transmitter power on the one hand, and the number of spacewave receivers to be served and the quality of the service that is acceptable on the other, would allow the use of a lower system margin. All that can be said in today's circumstances is that the total of 22 db is the author's present judgement of what is required, but that 22 db is so large a number -- each additional DC kilowatt costs about $5 million in initial system acquisition cost -- that its verification should be a serious challenge to professionals in the radiowave propagation and communications systems analysis communities. Indeed, the matter of excess radiowave path loss is fundamental to any consideration of employing the electromagnetic spectrum for a UHF DBS-A ~system, and it requires careful and comprehensive experimental and statistical study. Frequency Selective Fading There is little reason to imagine that any of the signal fading circumstances previously outlined would result in important frequency selective fading across the bandwidth of interest in this suggested system design. Other Considerations The use of sophisticated compandor-expandor techniques will provide an effective increase in the signal-to-voice ratio of some 16 db, and certainly should be employed. Advantage also should be taken of individual channel use statistics when a large number of channels are available: when any channel would not be in use, it is assumed that its subtransmitter RF power output would be turned off, or at least reduced by lO db or more. Under these circumstances statistical advantage can be taken of the fact that, on a second-to-second basis, all in-use audio channels are not in continuous use. When a large number of audio channels are being used, the tote] RF (and, therefore, DC) power required would be at least 5 db less than the amount calculated on the basis of multiplying the required power per channel by the number of channels. Caution should be exercised, however, in adding this 16 db and 5 db; it may be that the realizable total is closer to 18 db than to 2l ~ b. The number of independent channels that the space segments of any regional system would be designed to accommodate is assumed here to be 300. This number would allow each of about 100 countries to have access to one Standard service channel that could be broadcast to one or more standard surface areas simultaneously, and to provide a further capacity of 200 - 27 - WORKING PAPER

OCR for page 23
WORKING PAPER channels that would allow more than one simultaneous broadcast by many countries to any one space segment's service region. The maximum number of independent radiation beams provided by each such space segment would also be 300, allowing any broadcast to serve more than the one standard 10,000-square-mile area provided by each radiated beam. Without confident "service market" knowledge based upon study of the geographic, demographic, listening, and other characteristics of any region's potential listening audiences, it is not possible to estimate the maximum expected channel X beam product (i.e., the number of combinations and permutations of beams and channels) that could be in use simultaneously. It is assumed here--somewhat arbitrarily--that any regional system-service'c maximum usage product would be one-third to one-quarter the total number of such combinations and permutations of 300 channels X 300 beams = 90,000 channels X beams, i.e., 23,000 channels X beams. The tote] RF and therefore DC power required thus would be 5-6 db less than the amount calculated on the basis of multiplying the required power/channel by the total number of channels that, in principle, would be available for use. Again, as in the case of required system margin, this assumption must be given careful study by communications systems analysis professionals. Ci rcuit Qual ity Given the near complete absence of frequency selective fading (for other than mobile reception), co- and adjacent-channel signal interference, and the influence of atmospheric and external commercial-industrial electrical noise, the circuit communications quality is defined here as the product of t he sp acewave rece i ver' s post- detect i on s i gnal -to-no i se rat i o ~ S/ N ~ and i ts audio bandwidth. The circuit performance required to provide the three different service qualities are Basic Service, Standard Service, and Superior Service. Basic Service. For Basic Service the circuit performance would be 35 db in 5 kHz. This would be the minimum, systemwide performance expected with the minimum suggested receiver audio bandwidth, antenna and cost. Basic Service would provide a performance that should be acceptable in those relatively few, remote, low-popu~ation density areas that are served by few, if any, other attention-commanding electronic communications media. These areas probably would contain less than 0.] percent of the total population expected to be able to listen to DBS-A broadcasts. Standard Service. For Standard Service the circuit performance would be 45 db S/N in 5 kHz. This would be the performance expected in nearly all locations served by the system's space segments that should be achieved with the minimum receiver characteristics. Considering the 45 db S/N and the absence of electrical storm impulse noise, impulse noise from automobile ignition and other commercial and industrial sources, and skywave interference caused by distant, high-powered, co-channel and adjacent channel broadcasting transmitters, this quality of service would be at least equivalent to that obtainable by using a fine, modern receiver operated within the primary service area of a local, AM (MF) over-the-air - 28 - WORKI NG PAP ER

OCR for page 23
WORKING PAPER broadcasting station. The quality would closely approach that available in over-the-air FM (VHF) broadcasting. In considering these Standard Service characteristics it should be noted for comparison purposes that a fundamental design goal of the VOA s HE modernization and expansion program is to ...provide a 73 db tpost-detection] signal-to-noise density in a 1 Hz bandwidth at 90 percent reliability for 90 percent of the locations and hours.... _ A 73 db S/N in 1 Hz is the equivalent of 36 db in a 5 kHz bandwidth i.e. ~ db more than is adopted here for a Basic Service quality and 9 db less than that for a Standard Service quality. The comparable service goals for the reliability of a DBS-A system-service would be 99.9 percent for both locations and hours. For all practical purposes however it is impossible to design a surface-based HE real-time audio broadcasting system-service that would approach 99.9 percent/99.9 percent. The cost in effective radiated power spectrum and number of transmitters and transmitter sites required would be enormous and prohibitive even for a country with the resources of the United Statese And probably no system could deal effectively with sudden ionospheric disturbances (SID s). follows: In addition the received S/N required for satisfactory use of audio broadcasting signals need not be as high as those for signals used in a long-hau] trunk service. These broadcasting signals are used directly at the individual receiver s location and do not have to be retransmitted for use elsewhere and therefore they do not have to be designed to guard against the signal degradation associated with such subsequent retransmission. Superior Service. For Superior Service the circuit performance would be 50 db in 15 kHz. This quality of service would be available (at a user price higher than the Standard Service price) to any broadcaster who wished to reach an audience served by other competing high-qua~ity electronic communications media. This quality would be equivalent to that expected to be obtained by using a fine receiver within the primary service area of a U.S. local FM (VHF) broadcasting station. This service would be available on about lO percent of the system s channels at any one time. Fundamental Circuit Characteristics Related to the Power Budget The other more important illustrative circuit characteristics are as i. Frequency = 2.5 GHz. Gain of the space segment transmitting antenna GT = 62 db in the center of each beam assuming a reflector illumination efficiency factor of 60 percent. 16. Voice of America Engineering Standards VOA Standard - l6775.0l High Frequency (Shortwave) Broadcast System Design Chapter 1: Requirements Definition January 7 1985 page 2. - 29 WORKING PAPER

OCR for page 23
WORKING PAPER 3. KT F = 204 dbw/Hz. 4. Receiver noise figure (NF) = 5 db.~7 Receiver gain figure (GR) = 13 db. Frequency modulation (FM).~8 7. Required FM receiver threshold carrier-to-noise ratio referred to 25 kHz, (C/N) = lO db. 8. Free space loss, i.e., the inverse distance squared radiowave path loss between small doublet transmitting and receiving antennas that are very short relative to the radio wavelength, is 191 db along a vertical, subsatellite, path. 9. Ordinary system design and operating margin to accommodate receiver locations away from a beam center, and/or away from the subsatellite region i9 (for which the propagation path loss will be greater and, because the receiving antenna beam sees more of the Earth's surface, the noise temperature will be higher); less than maximum gain for beams formed by feeds offset far from the reflector axis, and longer-term equipment degradation = 7 db. lo. Loss in converting the space segment's DC power to transmitter RF power output, assuming solid state final stages, and including line losses from the final power amplifiers to the antenna feeds, power required for the power amplifiers' driver stages, power for the receiver to receive signals from the surface feeder station; power consumed by the switch; and general spacecraft housekeeping demands = 5 db. Space Segment Power Requirements Basic Service. Basic Service is defined as 35 db SIN in 5 kHz, post-detection. The individual RF channel width assumed is about 25 kHz. The space segment power requirements for the Basic Service, per channel, per standard coverage area, follow: 17. It is assumed that in mass production this low figure could be obtained at an acceptably low financial cost; some communications engineers argue for a much lower figure, perhaps as low as 2 db. lS. The alternative use of digital modulation certainly should also be considered. 19. The overall antenna pattern should be designed to favor higher latitude reception to some extent. - 30 - WORK ~ NO PAPER

OCR for page 23
i + GT = 62 db OR = 13 db KT F re- ferred to ~ Watt/Hz =204 dbw DC-to-RF conversion, etc. = 5 db Ordinary system margin = 7 db Free space path loss =191 db Channel use statistics = 5 db NF = 5 db Structure, foliage, rough terrain and ionosphere excess attenuation = 15 db 25 kHz referred to ~ Hz = 44 db FM Pre- emphasis effective S/N increase = 3 db C/N (in 25 kHz) = JO db Total: +287 db Total: -277 db The S/N = lO db ~C/N] + 16 db ~compandor-expandorJ20 + 9db ~10 logic (3) (~.6~2] = 35 db. WORKING PAPER Therefore, the DC power required is 277 db - 287 db = - lO db referred to ~ watt = 0.] watt DC/5 kHz channel/ standard unit surface area. The total DC power required for Basic Service for an entire regional system--one employing a single space segment with the maximum product of channels X the number of beams expected to be in use at any one time = 23,000--is (Owl (23,000) = 2,300 watts. Because a large frequency re-use factor would accompany the use of the type of space segment multiple-beam antenna suggested here, a total spectrum allocation of a few hundred kHz, perhaps as much as a MHz, would be required. 20. It is quite possible that (at long last) sophisticated speech processing techniques may begin to move out of the R&D laboratories in the next few years and be realized in low-cost receivers employing solid state integrated circuitry. If so, significant transmitter power reduction and system cost reductions could eventually occur. 31 WORKING PAPER

OCR for page 23
WORKING PAPER It should be noted that the requirement for a spectrum allocation of some 25 kHz/channe7 would be the total spectrum requirement/5 kHz post-detection audio channel serving any given area. In principle, HE broadcasters, employing double sideband AM, should require an allocation of lO kHz, i.e., less than half this amount. In practice, however, several transmitters are often employed simultaneously in order to maintain a desired S/N in the face of propagation and/or external noise vagaries. The absolute amount of spectrum use/program broadcast can exceed lO kHz by a significant amount. Indeed some broadcasters plan to broadcast a single program designed to reach an audience distributed throughout four time zones by using as many as six transmitters broadcasting simultaneously on different frequencies when circumstances warrant doing so. In that case, 60 kHz of spectrum would be used rather than 10 kHz. Standard Service. Standard Service is defined as 45 db S/N in 5 kHz, post-detection. The individual R.F. channel width assumed is 60 kHz. The power requirements for Standard Service, per channel, per standard coverage area follow: The channel width of 60 kHz = 48 db referred to ~ Hz (i.e., there would be 4 db more receiver input noise power than for the Basic Service). The S/N = lO db SCAN] + 16 db tcompandor-expandorj20 + 19db tl0 logy (3) (5.2~2] = 45 db. Therefore, the power required in the average space-segment = (O.~) (2.5) = 0.25 watt DC/5 kHz channel/standard unit surface area. For a 23,000 maximum in-use channel X beam product, the total DC power required to provide Standard Service is: (0.25) (23,000) = 5,800 watts. Some 2.5X the total spectrum allocation required for a Basic Service would be required for a Standard Service. The use of narrow radiation beams would result in small surface footprints and, with the use of FM and perhaps orthogonal polarization on adjacent beams, would allow an extraordinary amount of interference-free re-use. It would not be surprising if a UHF DBS-A system could provide services throughout the world with a total spectrum occupancy of no more than 2.5 MHz (see Superior Service quality following), compared to the total of some 3.0 MHz now placed at the disposal of HE international audio broadcasting, if 60 kHz/channe] were used to deliver a Standard quality of . service. Superior Service. Superior Service is defined as 50 db S/N in 15 kHz post-detection. The RF channel width assumed is 300 kHz. The power requirements, per channel, per standard coverage area, follow: - 32 - WORKING PAPER

OCR for page 23
WORKING PAPER The channel width of 300 kHz = 55 db referred to ~ Hz (i.e., there would be l] db more receiver input noise power than for the Basic Service). Provision for stereo also might be included. Recalling that OR = lS db (i.e., 5 db more than that assumed for the Basic and Standard Services) the net power increase required = + ll db - 5 db = + 6 db referred to the Basic Service. But only lO percent of the system channels are to have this additional capability and not all of these channels would be expected to be in use simultaneously. The system power increase referred to the Basic Service is therefore probably less than (0.~4), or perhaps less than ~ db. Thus the power required to provide a region with a Superior Service on lO percent of its channels, with the remaining 90 percent providing a Standard Service, is some 7,000 DC space segment watts. In view of all of the judgements, approximations and Woundings that must be accepted at this time, the peak space segment DC power level will be taken as 5 kilowatts DC. Al] three of these Services--Basic, Standard, and Superior--assume some modest receiver antenna gain. Although the antenna sizes would be small and the receivers could be light-weight and easily transported, they would have to be employed so that the antenna directivity remained essentially fixed in space while in use. If it were decided to provide a truly mobile service for all beams and channels at the space segment 5,000 DC watt level, the Basic Service could be provided with a receiving antenna gain of 9 db, i.e., over a received acceptance half-power beamwidth of about 80 with an efficiency factor of lOO percent. If it were required, instead, to operate a Basic quality mobile service (again, at the space segment power level of 5,000 DC watts, i.e., without increasing the space segment power) with a receiver antenna gain of -2 db, this service could be offered if the use were confined to JO percent of the system's channels. Given all the assumptions, the space segment DC power level estimate of 5 kilowatts should not be expected to be more accurate than an order of magnitude (+ 5 db). It is interesting to note that while this is a respectable satellite DC power level today it is about the DC power level required in a commercial, multibeam, geostationary satellite that would provide a microwave DBS-video (DBS-TV), i.e., direct television, service to a large (over a million square mile) surface area. NASA recently tested a satisfactory solar cell array in space on the Shuttle that, if it had included all of the solar cells it is capable of mounting, would have provided 12 kilowatts, i.e., the amount expected by Space Industries, Inc., to be in use by the end of 1989 at the first of its "space factories." And 5 kilowatts is only 5 percent of the initial power level of 100 kilowatts now planned for the Space Station a decade or so from now. International agreement now limits the RF power flux density generated by the transmitter of a space segment of any DBS system operating in the 2.5 GHz band. For radiation arriving over angles from the vertical (the subsatellite point) down to 25 above the horizon, no more than -137 WORKING PAPER

OCR for page 23
WORKING PAPER dbw/square meter/4 kHz band is allowed.2] The allowance is less for angles closer to the horizon. The average RF power expected here to be radiated by a UHf DBS-A FM system-service designed to provide a Standard Service in any 60 kHz channel over a 10,000 square mile area would be 5 db below the space segment's DC power of 0.] watt, i.e., 0.03 watt. This would be the equivalent of (0.03~4~/~60) = 0.002 watt in 4 kHz. Because the standard service area considered here is 10,000 square miles (i.e., 2.6 x 10~0 square meters), this would be a power flux density of (0.002~/~2.6 x 10~0) = ~ x 10~~4 = -132 dbw, i.e., 5 db more than is presently allowed. There should be some modest concern about meeting the specified surface power flux density if the maximum space transmitter RF power output, per channel, per beam, suggested here as a reasonable initial estimate is eventually seen to be the amount required. But because the required RF power output is not estimated here more accurately than an order of magnitude, a judgement that the power flux density limitation is sufficiently close to being able to be met is acceptable for the purposes of this paper. In any event, radio engineers concerned with the introduction of a new service understand that the identification of a surface flux density limitation is simply sound initial guidance arrived at by other engineers who were concerned with the introduction of an earlier service. The basic goal of the former is to see that any radio interference caused by one radio service to another is kept within acceptable limits. A number of engineering steps could be taken to see that this interference goal would be attained even though today's 2.5 GHz flux density limitations might be exceeded somewhat by a DBS-A system in some areas. Communications system analysis professionals should look into the actual present and planned UHF band occupancy by surface fixed services throughout the world in order to begin to make useful judgements about such power flux density matters. If the total RF spectrum devoted to a DBS-A system-serYice providing (23,000~/2~4) = 600 channels, worldwide, were to be 80% of that now devoted to surface-based HE shortwave (i.e. (0.~3.lMHz) = some 2.5 MHz) then, with 60 kHz used per channel to provide a Standard Service and 5 different 60 kHz allowances made to avoid mutual interference between adjacent beams or footprints, each footprint area could be served by as many as eight channels (i.e., (60 kHz)~5~) = 2.5 MHz). If some metropolitan areas were served by more than eight channels, however, either more spectrum would need to be allocated or for these areas less spectrum than 60 kHz/channe] could be used and increased space segment radiated power employed in order to maintain the Standard Service S/N (again, keeping maximum allowable surface flux density limits in mind). Because such circumstances would be realized in only a small fraction of the footprints, and because the peak DC power required/channel/beam is only 21. Footnote 2561, ITU Radio Regulations. - 34 - WORKING PAPER

OCR for page 23
WORKING PAPER 0.25 watt for Standard Service, a relatively small overall increase in space segment power (a ~ db increase in peak space segment power would be somewhat more than 1000 watts) could allow a large increase in the number of channels in those relatively few areas where they would be needed. Once the DBS-A service was in sufficiently broad use, consideration could be given by shortwave broadcasters to vacating some of the 3.] MHz now used at HE so it could be allocated for other purposes. Finally, the 2.5 MHz suggested here as the approximate amount of spectrum required for a UHF DBS-A worldwide broadcasting system-service might be compared with the total of over 21 MHz allocated in the United States for all local over-the-air AM and FM broadcasting. - 35 - WORKI NO PAP ER