This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 10 M

Year: Dec 2012

The uplink of the satellite is the one in which the earth station is transmitting the signal and the satellite is receiving it. The carrier to noise ratio for uplink is given as:

$[\frac{C}{N_0}]_U=[EIRP]_U+[G/T]_U-[Losses]_U-[k] …….(1)$

[ ] indicates decibel value

In the above equation the values to be used are the earth station EIRP, the satellite receiver feeder losses, and satellite receiver $\frac{G}{T}$. The free space loss and other losses which are frequency dependent are calculated for the uplink performance. The resulting carrier to noise density ratio given by equation (1) is that which appears at the satellite receiver.

Now the uplink equation can be modified in terms of

• Saturation flux density
• Earth station HPA

Saturation Flux density:

We know the travelling wave tube amplifier (TWTA) in a satellite transponder exhibits power output saturation. The flux density required at the receiving antenna to produce saturation of the TWTA is termed the saturation flux density.

If saturation flux density is specified in the link budget, one can calculate the required EIRP at the earth station.

Consider the flux density in terms of EIRP,

$ψ_M=\frac{EIRP}{4πr^2} ……………. (2)$

In decibel notation,

${[ψ_M]}=[EIRP]+10 log⁡(\frac{1}{4πr^2}) ……… (3)$

We know free space loss is given as

$-[FSL]=10 log⁡(\frac{λ^2}{4π})+10 log(\frac⁡{1}{4πr^2 }) ……. (4)$

Using equation (4) in equation (3)

$[ψ_M ]=[EIRP]-[FSL]-10 log⁡(\frac{λ^2}{4π})….... (5)$

The $\frac{λ^2}{4π}$term has dimensions of area and it is the effective area of an isotropic antenna. Denoting this by $A_0$ given as:

$[A_0 ]=10 log⁡(\frac{λ^2}{4π})………. (6)$

Combining equation (5) and (6) and rearranging the terms we get

$[EIRP]=[ψ_M ]+[FSL]+[A_0] ………… (7)$

The above equation is derived on the basis that only loss present was the spreading loss denoted by $[FSL]$. However other propagation losses are the atmospheric absorption loss, the polarization mismatch loss, and the misalignment loss.

$[EIRP]=[ψ_M ]+[A_0]+[FSL]+[AA]+[PL]+[AML] ….. (8)$

We know $[Losses]=[FSL]+[AA]+[PL]+[AML]+[RFL]$

Hence equation(8) becomes

$[EIRP]=[ψ_M ]+[A_0]+[Losses]-[RFL] …… (9)$

This is for clear sky conditions and gives minimum value of [EIRP] which the earth station must provide to produce a given flux density at the satellite. Normally the saturation flux density is specified. With saturation values denoted by subscript S in equation (9) we get

$[EIRP]_S=[ψ_M ]+[A_0]+[Losses]_U-[RFL] ….. (10)$

INPUT BACK OFF:

When a number of carriers are present simultaneously in the TWTA, the operating point must be backed off to a linear portion of the transfer characteristic to reduce the effect of intermodulation distortion. Such multiple carrier operation occurs with frequency division multiple access (FDMA). The earth station EIRP is to be reduced by the specified back off $(BO)$, resulting in the uplink value of

$[EIRP]_U=[EIRP_S ]_U-[BO]_i …. (11)$

Using equation (1), (10) and (11) we get

$[\frac{C}{N_0} ]_U=[EIRP_S ]_U-[BO]_i+[\frac{G}{T}]_U-[Losses]_U-[k]$

$[\frac{C}{N_0 }]_U=[ψ_M ]+[A_0]+[Losses]_U-[RFL] -[BO]_i+[\frac{G}{T}]_U-[Losses]_U-[k]$

$[\frac{C}{N_0} ]_U=[ψ_M ]+[A_0]-[RFL] -[BO]_i+[\frac{G}{T}]_U-[k]……. (12)$

The earth station HPA:

The earth station HPA has to supply the radiated power, plus the transmit feeder losses, denoted by TFL dB. These include waveguide, filter and coupler losses between the HPA output and the transmit antenna. The power output of HPA is given by.$[P_HPA=[EIRP]-[G_T ]+[TFL]$

The EIRP is given by equation (11)

The earth station may have to transmit multiple carriers and its output will also require backoff, denoted by [BO]_HPA. The earth station HPA must be rated for a saturation power output given by

$[P_HPA ]=[P_HPA ]+[BO]_HPA$

Uplink rain fade margin:

The $\frac{C}{N_0}$ calculations have been made for clear sky conditions. As we know fading of the signal can arise in a number of ways. In the Ku band, rainfall is most significant. It results in attenuation of the signal and increases in noise temperature, thus degrading the $\frac{C}{N_0}$ at the satellite in two ways. The uplink carrier power at the satellite must be held within close limits for certain modes of operation and some form of uplink control is necessary to compensate for rain fades.