To achieve this, correcting a satellite channel’s impairments is one particular area that needs attention. Think of it as trying to get hi-fi sound out of your stereo when your only option is lo-fi. If you turn up the volume, that sound gets distorted and almost unintelligible.
What can be done?
In 2005 ESA launched an activity under the Technology Research Programme (TRP) 5, called "Study of enhanced digital transmission techniques for Broadband Satellite Digital Transmissions (BSDT)". One of the findings of the study determined that the effects of satellite TWTA non-linearities on high order modulation can significantly reduce their attractiveness and therefore may require special compensation techniques.
Specific algorithms can be designed for ground equipment to compensate: pre-compensation in the transmitter (modulator) and equaliser in the receiver (demodulator). In other words, we counter balance on the ground with what will occur on the satellite allowing us to achieve the highest amplification gain without distortion, thus capitalising on a satellite’s capacity.
ARTES and link quality
Having demonstrated the potential performance improvements of such techniques, Newtec, a Belgian company, introduced the EqualinkTM feature in 2007 as part of their Azimuth product line. These modems have been continuously improved since then within the ARTES 4 “AziSat” project. With this enhanced modem, it is possible to make a point-to-point link by satellite up to 120 Mbit/s on a classical 36 MHz transponder.
Newtec Senior Engineer Guido Verfaillie explains: “The overall performance of the communication system is improved by applying the described method of pre-distortion since the satellite TWTA is now operated closer to saturation and the receiver degradation due to the non-linear channel is decreased at the optimum operating point. The pre-distortion tables are calculated from given AM-AM and AM-PM characteristics (via web interface) and stored in the modulator.”
In the case of non-linearised TWTA, the following gains can be obtained:
For 32APSK modulation, there is typically a 2.2 dB improvement in the downlink (OBO + receiver degradation) and 3.5 dB less IBO (for a given saturation flux density this will improve the uplink and possibly the total link budget)
For 16APSK modulation, improvement is typically 1.2 dB on the downlink and 2 dB less IBO. Figures are somewhat dependent on the coding used and on the insertion of pilot symbols.
These techniques have also been successfully implemented in the Newtec Sat3Play system, also developed under the ARTES 3-4 programme. The Sat3Play terminals are used to offer consumer Internet access, telephony and TV services by satellite to European consumers at a starting price of 30 € per month.
