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Adaptive Coding and Modulation (ACM)


What is Adaptive Coding and Modulation (ACM)?

ACM or Adaptive Coding and Modulation is a technology which can automatically change the forward error correction and modulation of a link to compensate for changes in link conditions. Commonly these changes are due to weather, e.g. rain fade, but can also come from other sources such as RF level changes or interference.

How ACM Works

ACM has been used for some time in wireless communications, including terrestrial microwave applications. Only more recently has it been used over satellite links. ACM optimizes the throughput in a wireless data link, by adapting the modulation order and the Forward Error Correction code rate according to the noise conditions on the link.

In an ACM link, the symbol rate (and power) of the wireless communication system remain constant. This ensures that the bandwidth allocated for a particular link is never exceeded. Assuming that the symbol rate does not change, if modulation and coding are changed, then the data rate will change.

Benefits of using Adaptive Coding and Modulation


ACM Maximizes Link Throughput

ACM enables the operators to increase link availability and throughput simultaneously by dynamically adjusting the link to a more robust MODCOD in fade conditions and a higher throughput in clear sky conditions.

ACM Maximizes Link Availability

Traditionally, pointing errors, noise and interference, inclined orbit satellite operation and rain fade can all degrade satellite link performance. Satellite link designers have traditionally relied on worst case link margin to overcome those impairments which led to significant inefficiencies. By utilizing ACM, satellite networks are now able to utilize that link margin which was previously there to ensure link availability in poor conditions.

An Example of ACM

The image below shows what your link looks as weather affects the signal. With ACM, we gain the benefit of maximum throughput when skies are clear, and maximum availability when the weather could affect our signal.

The following equations

Symbol rate = bit rate ÷ (modulation order × code rate)

For example, for Rate 3/4 QPSK (where modulation order = 2) Symbol rate = bit rate * 0.666

Re-arranging:

Bit rate = symbol rate × modulation order × code rate

So, in changing to a higher modulation order or code rate , the bit rate is increased, and in changing to a lower modulation order or code rate, the bit rate is decreased.



Additional References