ND SatCom SKYWAN IDU 7000, Руководство пользователя

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2010-10-26 Network Design and Engineering Guide 79
Outdoor Unit and Satellite Link Design
Fundamentals of Link Budget Calculation
bandwidth. As long as the summary PEB for all carriers is still smaller than the summary band-
width of all carriers, no extra space segment has to be leased for the carriers with high power
requirement.
Example :
Available transponder EIRP: 50 dBW
Transponder bandwidth: 36 MHz
Required carrier EIRP: 40 dBW
-> Carrier PEB: 3.6 MHz
For an example we assume the values given above. Now we make a calculation for a Sky-
WAN
®
carrier with a modem rate of 1000 kbps, QPSK modulation and 1/3 coding. We find a
carrier bandwidth of 1904 kHz (cf. previous example) and a power requirement for the satellite
of 30 dBW. Using the results from the previous example, we can now derive the required band-
width and PEB for other modulation and coding selections.
Table 3-3 Relation between Modulation, Coding and Carrier PEB
In this example, with QPSK 1/3 coding the carrier uses a much smaller fraction of the trans-
ponder’s power than the transponder’s bandwidth. Increasing the coding will increase the pow-
er requirement and decrease the carrier bandwidth. Indeed, at QPSK - 6/7 the satellite link
power and bandwidth is optimally equilibrated as the carrier bandwidth and PEB are identical.
Any further increase in modulation and coding will increase again the space segment cost, as
the PEB increases even though the carrier bandwidth would still decrease. So the optimal mod-
ulation and coding for this downlink would be QPSK - 6/7.
Rain fade
In Ku-Band strong rain falls can attenuate the signal substantially resulting in a temporary drop
of the Eb/No levels below the minimum requirement for a good quality satellite link. This may
lead to a temporary outage of services. To prevent this the link budget has to include a rain
margin for the satellite link power requirement. The amount of this margin depends on:
- The probability of intense rain at the geographical position of the earth station.
- The required availability for the satellite link.
- The carrier frequency.
As the rain fade increases with the carrier frequency, it is basically negligible for C-Band but
has to be taken into account for Ku-Band or higher carrier frequencies. The probability of in-
tense rain can be derived from the ITU-T rain zones which represent the maximum rain inten-
sity at a specific location which is not exceeded in 99.9% of a typical year. Rain zones A (polar
and desert regions) to Q (tropical Africa) are defined by the ITU-T. Finally, a required satellite
Modulation
- Coding Carrier
EIRP on
Satellite
[dBW] PEB [kHz] Carrier Band-
width [kHz] Space Segment to
lease on Transpond-
er [kHz]
QPSK - 1/3 30.0 360 1904 1904
QPSK - 6/7 33.2 758 758 758
8PSK - 6/7 36.4 1571 428 1571