What is a Link Analysis and What Can it Tell Me?
The definition of a satellite transmission can be summarized as the successful delivery of information from one part of the Earth to another using a satellite repeater in space.
A link analysis (also known as a link budget) is a theoretical mathematic model of how a satellite circuit should work. It is comprised of known and unknown values that must be assumed. Obviously, a link analysis is only as good as the information used in the analysis process. It is for this reason that both Intelsat engineers and Intelsat customers must have a full understanding of the elements involved to properly model a proposed service.
To begin, one must be familiar with the elements. A satellite circuit is complex and made up of many parts, including, but not limited to:
- a satellite transmit antenna
- a High Power Amplifier (HPA)
- a satellite receiver
- a transponder gain setting
- a transponder HPA
- a receive earth station antenna, and
- a Low Noise Block (LNB) converter.
Each part must be set up per recommended specifications, and within their operating limits, to ensure the overall circuit performance is optimal for the successful transmission of the information.
The following information is needed from Intelsat customers to ensure an accurate link analysis:
- requested satellite and transponder,
- downlink antenna size, and location, and
- uplink antenna and HPA size, and location.
Also needed for accurate link analyses are carrier parameters, which include:
- Data Information Rate,
- Modulation: QPSK, 8PSK, 16QAM, etc.,
- Forward Error Correction (FEC): 1/2, 2/3, 3/4, 5/6, 7/8, etc.,
- Outer coding schemes, such as Reed Solomon or other including DVB or DVBS2, and
- Acceptable Operational EB/No Threshold.*
*This is crucial as it sets the circuit availability target.
There is information, such as future transponder loading, adjacent satellite interference (ASI), etc., for which the exact values are not known exactly because they are dynamic and change over time.
The challenge is to capture a reasonable assumption of an acceptable value. One of two paths are generally pursued. One, a more conservative approach, presumes the worst possible condition the link might face. This “worst case scenario” analysis ensures a circuit will work under the worst possible conditions. However, the cost to a customer for extra power (and possibly unnecessary) might be economically disadvantageous.
The more pragmatic path to take is to provide an economically sensitive, and more realistic, value that is noted and/or agreed upon in advance. This is why the engineer creating the link analysis must understand the need and use of the proposed circuit.
There are many ways to perform a link analysis. When possible, it is preferable to begin with the factors related to the receive earth station. The carrier parameters are considered next. This determines the optimal settings to make transponder bandwidth equal to the transponder power needed for the circuit. Then, the work continues backwards to determine the optimal transmit antenna and the size of the high powered amplifier (HPA) needed to transmit the information to the satellite and back to the receive earth station.
Unless otherwise agreed upon, Intelsat’s normal, annual availability assumption for Ku-band is 99.6% and for C-band is 99.96%. However, it is important that the value is known and agreed upon in advance to meet customer preference.
When referring to balanced services, one means the transponder bandwidth needed equals the transponder power needed to make the circuit successful. However, there are other circumstances that might not make that possible. In an instance where a user has a transmit HPA limitation of 75 Watts, one must make the carrier need less power than bandwidth to keep the transmit earth station HPA power under 75 Watts.
To summarize this link analysis example, using Galaxy 17, transponder 19K, resource of 12 MHz of transponder bandwidth and 4.4 MHz of transponder power, a 1.8 meter earth station at a remote location in Miami, using 51.4 Watts of HPA power, can transmit a 20 Megabit-per-second carrier—using the carrier parameters provided in the link analysis—to a 4.5 meter earth station located at the Miami site at a circuit availability of 99% annually.
Given that link analyses are only as good as the information used to create them, they should be expected to gauge circuit performance under the assumed working parameters in the analysis. They are used as models when looking for solutions to a particular challenge, before committing to spending capital resources.
The greater the amount of “known” information loaded into the analysis, the more accurate it will be. When performing a link analysis, Intelsat’s customers work with an Intelsat Customer Solutions Engineer to ensure all assumptions are acceptable and a complete understanding of the performance of the circuit is achieved.