Page 387 - Satellite Communications, Fourth Edition

The Space Link 367

and therefore

c C d c C d [B N ] (12.37)
N 0 N

[C/N] is a true power ratio in units of decibels, and [B ] is in decibels
N
relative to 1 Hz, or dBHz. Thus, the units for [C/N ] are dBHz.
0
Substituting Eq. (12.37) for [C/N] gives
c C d [EIRP] c G d [LOSSES] [k] (12.38)
N 0 T

Example 12.9 In a link-budget calculation at 12 GHz, the free-space loss is 206
dB, the antenna pointing loss is 1 dB, and the atmospheric absorption is 2 dB. The
receiver [G/T] is 19.5 dB/K, and receiver feeder losses are 1 dB. The EIRP is 48
dBW. Calculate the carrier-to-noise spectral density ratio.
Solution The data are best presented in tabular form and in fact lend themselves
readily to spreadsheet-type computations. For brevity, the units are shown as
decilogs, (see App. G) and losses are entered as negative numbers to take account
of the minus sign in Eq. (12.38). Recall that Boltzmann’s constant equates to
228.6 decilogs, so [k] 228.6 decilogs, as shown in the following table.
Entering data in this way allows the final result to be entered in a
table cell as the sum of the terms in the rows above the cell, a feature
usually incorporated in spreadsheets and word processors. This is illus-
trated in the following table.

Quantity Decilogs
Free-space loss 206
Atmospheric absorption loss 2
Antenna pointing loss 1
Receiver feeder losses 1
Polarization mismatch loss 0
Receiver G/T ratio 19.5
EIRP 48
[k] 228.6
[C/N 0 ], Eq. (12.38) 86.1

The final result, 86.10 dBHz, is the algebraic sum of the quantities
as given in Eq. (12.38).

12.7 The Uplink
The uplink of a satellite circuit is the one in which the earth station is
transmitting the signal and the satellite is receiving it. Equation (12.38)
can be applied to the uplink, but subscript U will be used to denote