Space Acquisitions

Link Budget

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A Link Budget shows all of the gains and losses from a transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication system. It’s used to predict the performance of a transmitter and receiver communication link to show in advance if its performance is acceptable, or if one option is better than another. It accounts for the attenuation of the transmitted signal due to propagation, as well as the antenna gains, feedline, miscellaneous losses and added margin. [1]

A simple link budget equation looks like this:

Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses (dB)

The Information that is needed to perform a Link Budget include:

  • The saturated EIRP and saturated flux density of the transponder.
  • The satellite G/T figure appropriate to your planned uplink location.
  • Satellite transponder bandwidth.
  • Satellite transponder output backoff or attenuation.
  • Satellite transponder input backoff or attenuation.

You will also need the following information that you and your customer can supply:

  • Latitude and longitude of the uplink and downlink earth stations.
  • Planned data or information rate.
  • Modulation type (BPSK or QPSK)
  • Forward error correction rate (1/2 or 3/4)
  • Spread Factor – if any (use only for spread spectrum systems)
  • Uplink and Downlink frequencies.
  • Uplink and Downlink antenna sizes.
  • Uplink and Downlink antenna efficiency.
  • Uplink and Downlink transmit and receive gains at frequency.
  • Minimum digital signal strength (EB/No) for desired Bit Error Rate (BER) performance.

*The above information can generally be obtained from the satellite operator and customer.

Often link budget equations can become messy and complex, so there have evolved some standard practices to simplify the link budget equation

  • The wavelength term is often considered part of the free space loss equation. This complexity reduction is acceptable for terrestrial communication systems, where only line of sight is considered.
  • Considering all carrier wave propagation to be wavelength-independent. This is justified by the conservation of energy law that requires that the electric field decrease in power as the square of the distance regardless of frequency (in free space propagation conditions).

Link Budget Evaluation

  1. Has anything significant been left out?
    1. This only comes with experience, yours or someone you trust.
  2. Are any values significantly different to other similar budgets?
    1. If so, why?
    2. Is the difference explained?
  3. Remember the creator must often guess at knowledge level of the audience.
    1. Be willing to ask questions.

AcqTips:

  • Link Budget information can generally be obtained from the satellite operator and customer.

AcqLinks and References:

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