7/26/2023 0 Comments Downlink definition![]() ![]() Return channels can operate at higher data rates in combination with HTS high uplink G/T, and with modest EIRP, are now bumping into symbol rate limits. The very capable iDirect Evolution 8000 Airborne Router Modem has a 45 Msps forward (DVB-S2/ACM) and a 7.5 Msps return (D-TDMA) symbol rate limit.įorward channel symbol rate limits were not much of a concern until the prospect of HTS with very wide transponders: from 36 MHz to 72 MHz to 125 MHz onto 500 MHz. IDirect Evolution 8000 Airborne Router Modem Features The correlation to a given modulation and coding to signal strength is a matter of propriety and competition (hence my fabrication above, which follows broadly DVB-S2 claims). The following chart reflects a fabricated modem that demonstrates how the modulation choices improve with signal, and to appreciate that coding gains vary across the range of a given modulation.Īdaptive Modulation Options for a generic modem In order to gain enough energy per bit with poor signal strength, less efficient coding and modulations, and even spread spectrum (multiplying) is necessary to dig out a usable signal. The Shannon Limit remains the barrier from which we can only approach. Spectral efficiency can be related to signal strength, and to the best of my knowledge Ultimately, the modem is deriving some measure of Eb/No in order to demodulate the bit, and this is the baseline performance goal. One converts to Energy per Bit (Eb/No) by applying a modulation factor to signal strength. I am currently modeling 18% (of the symbol rate) guard band, but I have seen implementation with over 30% guard band.Ĭarrier to Noise plus interference (dB) is a reflection of the composite signal present, to which I will generically refer to as signal strength. The guard band is a matter of competitiveness, as its utility does not translate into useful symbol rate. available bandwidth (in the transponder).There are hard limits to spectrum committed due to: Spectral efficiency is impacted inversely by spreading factors. The spectrum committed and the guard band are proportional to the spreading factor times the symbol rate. The spreading factor is simply a means to boost or suppress a signal. The modulated symbol rate can be multiplied by a spreading factor. The spectrum committedor occupied includes the resultant symbol rate (symbols per second) and any guard band between channels. Scaling spectrum is spectral efficiency, or the data rate (bits per second) achieved from the spectrum committed (Hz). Ultimately, there are limits the power and spectrum that can be applied. ![]() The forward and return channels operate using very different characteristics and limitations. This involves an uplink from the remote terminal to the satellite, and a downlink from the satellite to the teleport.įorward and Return Channels using an Airborne Remote Terminal (Aircraft Earth Station) The return channel is the path from the remote terminal to the ground. This involves an uplink from the teleport to the satellite, and a downlink from the satellite to the remote terminal. The forward channel is the path from the ground to a remote terminal. ![]()
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