Satellite Link Budget¶

In this section we are going to explain what is a satellite link budget and what are the main parameters it involves.

Satellite Communication System¶

Assuming that a telecommunication system is composed by a transmitter a medium and a receiver, the link budget is the accounting of all gain and losses in the power signal from the transmitter, passing through the medium, to the receiver.So, the link budget analyse how much power is lost in the signal path, and can discriminate the loss power values of each cause of loss.

To easily understand, we can analyse the satellite system showed in figure 1. Considering a signal one-way-travel, and just analyzing the individual uplink path, the link budget will perform the power received by the satellite taking in account the amplification and the attenuation of the link.The link, in this case, is composed by the transmitter: the Earth Station, the media: the atmosphere and free space ,and the receiver: the Satellite. Then, in the link budget calculating, we taking in account the following losses and gains:

• Gain of the transmitter and receiver amplifier.
• Attenuation in the atmosphere propagation.
• Depointing losses.
• Polarization mismatch losses.
• Losses in transmitting and receiving equipment.

 
<div style="margin-left: auto; margin-right: auto; width: 50em">Figure 2 - Gain and losses from a signal travelling from a transmitter to a receiver, considering individual uplink path. P is the power before the transmitter amplifier, Gain is the transmitter amplifier gain, PR is the power received, Loss 1 the power lost due to the transmitter equipment, Loss 2 the power lost due to the atmosphere effects, Loss 3 the power lost due to the receiver equipment.
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To evaluate the quality of our link, one important measure is the Bit Error Rate, $BER$. If we want that our signal maintain a low $BER$ we must to start with a higher power and make sure that even with all losses along the path way, we'll have enough power available at each point to keep this $BER$ value.The $BER$ is a function of the bit energy per noise-density of the signal $\frac{Eb}{No}$. The ratio between $BER$ and $\frac{Eb}{No}$ depends on the digital modulating implemented. An example of $BER$ x $\frac{Eb}{No}$ graphic is showed in figure 3. Higher $\frac{Eb}{No}$ means better quality.

 
<div style="text-align: center;">Figure 3 - Exemples of $BER$ x $\frac{Eb}{No}$ graphics. M-PSK constellations.</div>

Other important parameters used to consider in link budget analysis performance are:

• Link performance: Carrier-to-noise power ratio $$ \frac{C}{No} $$

It's a measure of the power received by receiving equipment C, and the noise power density, introduced in the transmission medium. It's is also a measure of the quality of modulated carrier in the receiver input.

• Transmit equipment performance towards receiver: Effective isotropic radiated power $$EIRP$$
  Is a measure of the power $P_T$ and the performance of the transmit equipment in the direction of the receiver. It is expressed in dBW.

• Receiving equipment performance:_Figure of merit_ $$G/T$$

The figure of merit is expressed by a ratio between the maximum gain of a receiving antenna to the system equivalent noise temperature.In general it is expressed in dB/K. This relation is also a measure of the ability of the receiver to receive a signal with good quality (high carrier-to-noise ratio).

The figure 4 shows the detailed parameters involved in a satellite link.

 
<div style="text-align: center;">Figure 4 - Link configuration.</div>

tip.
To fully understand what is the link budget first we must understand how it is formed a satellite communication system. A satellite system is composed of three operational components: the ground segment, the space segment and the control segment.

• The space segment contains the satellite itself (or satellite constellation) and the uplink and downlink satellite links.
• The ground segment consists of all the traffic earth stations.
• The control segment consists of the ground facilities and systems for control and monitoring the satellite.