18. Interpreting the Results

Having obtained the output of our link budget, we now need to review the results and assess the viability of the link – if it has “closed”, whether the margins are adequate, and consider other parameters such as HPA sizing, antenna sizing (both uplink & downlink), availability etc.

Watch the video tutorial, and then look at the explanations below.

Closed Captions are available

Tutorial Video Summary

In this video, you have learnt the following:

  • How to analyse and understand the significance of initial Link Budget principal results
  • The significance of link budget being Power-limited vs. Bandwidth-limited
  • The importance of Excess Margins
  • The definition of Overall Availability
  • The importance of Bandwidth Occupancy
  • The meaning of Power Equivalent Bandwidth (PEB)
  • The significance of Link Efficiency

Link Budget – Power-limited vs. Bandwidth-limited 

The transponder has a total finite bandwidth available, and a signal is said to be bandwidth-limited where the available transponder bandwidth that can be allocated to the carrier is insufficient to meet the link target Es/No or Eb/No threshold.

Similarly, the transponder has a total finite power available, and a signal is said to be power-limited where the available transponder power (EIRP) for the carrier is insufficient to meet the carrier target C/N threshold for the required bandwidth.

Ideally the power and bandwidth demand should be adjusted to be perfectly balanced (1:1), but that is not realistically achievable.

In other words, as services are assigned to a transponder, there may be power available, but the transponder may run out of bandwidth (bandwidth-limited). In bandwidth-limited channels a more spectrally efficient modulation scheme is required to save bandwidth (e.g. using 8-PSK rather than QPSK).

Alternatively there may be bandwidth available, but the transponder does not have enough power to support the service in balance with all the other services in the transponder (power-limited). For a power-limited channel, the desired system performance is obtained with the smallest possible power.  So coding schemes are generally adapted to save power at the expense of bandwidth e.g. by increasing FEC and therefore the bandwidth requirement.

The primary objective of spectrally-efficient modulation is to maximize bandwidth efficiency, defined as the ratio of data rate to channel bandwidth (bits/second/Hz, or bps/Hz)

Power Equivalent Bandwidth

Total Power Equivalent Bandwidth (PEB) relates to the amount of total power used by the carrier represented as a bandwidth equivalent. The satellite operator will always charge for the larger of either Total Allocated Bandwidth or Total PEB.

So if bandwidth-limited, the lease charge will based on the Total Allocated Bandwidth; if power-limited, the charge will be based on the PEB value.

Overall Availability

In the tutorial video we saw how to calculate Overall availability.

The calculator shown here allows you to experiment with different availabilities.

Remember you can always export the link budget results in CSV format using the Copy to Clipboard button on the toolbar; or using the File menu, save the results as a Tabbed text or HTML file.

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