Predictive ACM Margin for DVB-S2 Modems and EPM IP Modem 21e for the Ka and EHF Bands

摘要

The French MoD has already deployed an Electronic Protected Modem (Thales's EPM Modem 21e) fitted with an IP interface. This modem is designed to provide an efficient use of the EHF resources (44/21 GHz). Contrary to the SHF concept of operation where links are planned with constant data rates and static margins, the EHF band requires the use of Fading Mitigation Techniques such as Adaptive Coding and Modulation (ACM) to adapt to propagation conditions and thus to reduce the link margins. The main drawback of this technique is to take an important margin (about 4 dB in our case) due to the feedback latency of this concept. This margin is mandatory to face with the worst case (for instance a fading slope of 1 dB/s and a latency of 3.5s lead to a margin of (1*3.5=) 3.5 dB). In clear sky or in stable conditions this margin is useless, the modem could trade it for increased capacity. In order to reduce this margin, we have added a prediction function to the ACM functionality of the modem in order to provide a gain in: Data rate (and consequently in bandwidth usage), System availability. The French MOD has conducted tests on two kinds of Satcom IP modems with different EHF propagation profiles: The EPM-IP Modem 21e from Thales, The DVB-S2 modem from Thales Alenia Space. The test results demonstrate that the prediction function provides a significant gain in transmitted data rates: - From 11% to 78 % with the EPM-IP Modem 21e, - From 3.5% to 13% with the DVB-S2 modem