Better availability. Higher spectral efficiency.
Adaptive Coding and Modulation (ACM) is a technology that automatically optimizes modulation and coding parameters to adapt to changing conditions of the radio link. Link adaptation is dynamic, where the signal parameters adjust as the radio link conditions change. Depending on the geographical region and link budget criteria, fading conditions typically occur during a very small percentage of time.
Most of the time, the SNR of the link is much higher than the operational SNR assumed by the configuration. This is usually referred to as the Link Margin. ACM can take advantage of this link margin by changing the modulation level during low fading conditions, thus, allowing the link to have a higher throughput during these periods.
ACM ensures that the highest possible modulation is used to fit the link’s actual current fading conditions. If fading occurs on link, the system automatically lowers the modulation level in order to avoid errors. When the link fading ends, the system automatically increases modulation to the highest possible level.
A modem supporting optional ACM can offer substantial operational improvements:
- ACM can turn fade margin into increased link capacity – substantial capacity improvement is possible, when compared to Constant Coding and Modulation (CCM) performance. While CCM constantly maintains a link margin for the worst link conditions, ACM will utilize this access link margin to be used in good link conditions to provide higher capacity by keeping a constant margin from the actual link condition.
- ACM can maximize the throughput regardless of the actual link conditions (noise, clear sky, rain fade, or other degradation factors). Initial setup is easy and requires no further user intervention.
- ACM can be set to a target link margin value which will always be maintained.
- With a traditional CCM system, severe degradation may cause the total loss of the link, actually resulting in zero throughput. ACM can keep the link active during heavy fades with lower throughput, and can yield much higher system availability.
For example, a satellite link, based on the NovelSat NS4 waveform, when planned for a CCM link fitting an 8dB CNR with 99.9% link availability, will require additional 4dB margin and will operate at 16APSK 19/30, providing 83.88Mbps over 36MHz transponder. The same link planned for ACM with 0.4dB margin, will be able to reach up to 32APSK 32/45, delivering 117.22Mbps in clear sky condition on the same transponder for 95% of the year, and go down to 16APSK 19/30 during the worst fading, providing a total of 99.9% link availability.
NovelSat Advanced ACM
For links with fast fading changes, for example when working in Ka band, activating Advanced ACM will enable the ACM mechanism to react faster to rapid changes in the link conditions up to 1dB/sec, by initiating ACM messages when the demodulator detects a dramatic change in SNR signal.
NovelSat ACM is offered in Different Forwarding Modes
- ACM in Layer 2 Transparent Forwarding Mode
- ACM in Layer 2 VLAN Switching Forwarding Mode
- ACM in Layer 3 IP Routing Forwarding Mode
- Constant Power Mode
- Constant Envelope Mode