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(12.5 0.5
Since circular polarization is used, the coefficients are found from Eq. (4.8): From Eq. (4.8a): a h 1 av 0.0218 1 0.0196 5 5 0.0207 2 2
ac 5
From Eq. (4.8b): ah # bh1av # bv 0.0218 3 1.207 1 0.0196 3 1.188 5 5 1.198 2 # ac 2 3 0.0207
bc 5
Using the Method 3 curves in Fig. 4.4 for p 0.01 percent and earth-station latitude 45 , the rain height is approximately 3.5 km, and as stated in the problem, at mean sea level ho 0 From Eq. (4.4): hR h0 3.5 sin 37
LS From Eq. (4.6) LG
5.82 km
sin El
LS cos El
cos 37
4.64 km
Sixteen
From Table 4.3: r01 From Eq. (4.5): L From Eq. (4.2): a 5 acRbc 5 0.0207 3 421.198 5 1.819 01 From Eq. (4.3): [A01] L 1.819 4.82 8.76 dB LS # r01 5.82 .829 4.82 90 90 4LG 0.829
The effect of rain is calculated as shown in Sec. 12.9.2. This requires the attenuation to be expressed as a power ratio A 5 10[A01]/10 5 100.876 5 7.52 For Eq. (12.60), the noise-to-signal ratios are required. From Example 16.1, [C/N0] 86.3 dBHz for clear sky, hence, a C b 5 10286.3/10 5 2.34 3 1029 N0 CS
The system noise temperature under clear-sky conditions is just TS, but the subscript will be changed to conform with Eq. (12.60): TSCS TS 170 K From Eq. (12.60): a N0 C b
rain
N0 C
1) #
Ta TSCS
b 272 b 170
5 2.34 3 1029 a7.52 1 6.52 3 5 4.21 3 1028 Hence, c
C d 5 10 log (4.21 3 1028)21 5 73.76 dBHz N0 rain
Recalculating the [Eb/N0] ratio, Eq. (10.24) gives: c Eb N0 d
rain
C N0
rain
[Rb]
2.26 dB
Direct Broadcast Satellite (DBS) Television
Thus the rain will completely wipe out the signal for 0.01 percent of the time. It is left as an exercise for the reader to find the size of antenna that would provide an adequate signal under these rain conditions.
16.12 Uplink Ground stations that provide the uplink signals to the satellites in a DBS system are highly complex systems in themselves, utilizing a wide range of receiving, recording, encoding, and transmission equipment. Signals will originate from many sources. Some will be analog TV received from satellite broadcasts. Others will originate in a studio, others from video cassette recordings, and some will be brought in on cable or optical fiber. Data signals and audio broadcast material also may be included. All of these must be converted to a uniform digital format, compressed, and time division multiplexed. Necessary service additions which must be part of the multiplexed stream are the program guide and conditional access. FEC is added to the bit stream, which is then used to QPSK modulate the carrier for a given transponder. The whole process, of course, is duplicated for each transponder carrier. Because of the complexity, the uplink facilities are concentrated at single locations specific to each broadcast company. The uplink facilities for Echostar s DISH network are shown in Fig. 16.9. The four uplink
Figure 16.9 Uplink facilities for Echostar s DISH network. (Courtesy of Echostar, at http://www.dishnetwork.com/content/aboutus/presskit/print_satellites/index.shtml)
Sixteen
TABLE 16.3
Uplink Facilities in Operation as of 1996 Location Oxford, Connecticut Castle Rock, Colorado Cheyenne, Wyoming Oakdale, Minnesota
Company AlphaStar DirecTV EchoStar U.S. Satellite Broadcasting
facilities in operation as of 1996, as given in Mead (2000), are shown in Table 16.3. 16.13 High De nition Television (HDTV) Table 16.1 shows the 18 advanced television systems committee (ATSC) formats for digital television, which includes HDTV. Some of the early HDTV systems were analog, and a description of these will be found in Kuhn, (1995). However, all analog TV transmissions in the U.S. are scheduled to shut down by January 1, 2009. It will be possible to get set-top boxes that convert HDTV signals to a format suitable for analog sets, but the high resolution, and many of the other advantages of digital TV will be lost in this process. In Europe, Astra is the major satellite provider, and they, along with 60 European broadcasters have agreed to standardize on two HDTV formats: 720p50 meaning 720 lines (or pixels) of vertical resolution, with progressive scan, at a 50 Hz refresh rate and 1080i25, meaning 1080 lines (or pixels) of vertical resolution, with interlace scan, at a 25 Hz refresh rate. The refresh rates are normally tied in with the frequency of the domestic electricity supply, 60 Hz in the U.S. and most of north America, and 50 Hz in Europe. In Table 16.1 a refresh rate of 24 Hz is listed, to make this format compatible with film projection at 24 frames per second. DirecTV plans to use H.264/AVC (MPEG-4 Part 10, see Sec. 16.7) in its HDTV satellite broadcasts and all HDTV services in Europe are expected to use this rather than the MPEG-2. Comparing H.264/AVC to MPEG-2 the bit rate reduction can vary between about 50 (Mark, 2005) to 65 percent (Wipro, 2004). Two high definition channels require a bit rate of 16 to 18 Mbps with MPEG-2 and 6 to 8 Mbps with H.264/AVC. As explained in Sec. 16.7, H.264/AVC is not backward compatible with MPEG-2, and this may mean considerable expense for the consumer who wishes to receive HDTV along with SDTV. The MPEG is working on making H.264/AVC compatible with MPEG-2 (Koenen, 1999).
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