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The uplink of a satellite circuit is the one in which the earth station is transmitting the signal and the satellite is receiving it specifically that the uplink is being considered.
In this Eq the values to be used are the earth station EIRP, the satellite receiver feeder losses, and satellite receiver G/T. The free-space loss and other losses which are frequency-dependent are calculated for the uplink frequency.
1. Input backoff
Number of carriers are present simultaneously in a TWTA, the operating point must be backed off to a linear portion of the transfer characteristic to reduce the effects of inter modulation distortion. Such multiple carrier operation occurs with frequency- division multiple access (FDMA), which is described in Chap. 14. The point to be made here is that backoff (BO) must be allowed for in the link- budget calculations.
Suppose that the saturation flux density for single-carrier operation is known. Input BO will be specified for multiple-carrier operation, referred to the single- carrier saturation level. The earth-station EIRP will have to be reduced by the specified BO, resulting in an uplink value of
[EIRP]U = [EIRPS]U + [BO]i
2. The earth station HPA
The earth station HPA has to supply the radiated power plus the transmit feeder losses, denoted here by TFL, or [TFL] dB. These include waveguide, filter, and coupler losses between the HPA output and the transmit antenna. Referring back to Eq. (12.3), the power output of the earth station itself may have to transmit multiple carriers, and its output also will require back off, denoted by [BO]HPA. The earth station HPA must be rated for a saturation power output given by
[PHPA,sat] = [PHPA] + [BO]HPA
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