MODULATION AND MULTIPLEXING TECHNIQUES Each earth station will, in general, be transmitting and receiving many messages simultaneously to and from a satellite. The messages may be 'phone calls, ratio and TV signals and/or computer signals. They are transmitted by modulating a carrier signal in some way - AM,FM, PM (analogue), or ASK, FSK, PSK etc (digital). In a multicarrier system the different messages are combined for transmission by multiplexing. The converse process of demultiplexing is carried out at the receiver The multiplexing techniques used are : i)Frequency Division Multiplexing (FDM) – each message is placed in a different frequency range by modulating a different carrier frequency. All the messages are combined for transmission.Each satellite link will have a certain bandwidth. The bandwidth may be divided into sub-bands with different sub-bands assigned to each earth station. The figure below shows a set of satellite transponders for (a) a C band and (b) a Ku band system.

The C band transponder uses a single down converter (D/C) and signal processing at 4GHz, The Ku band system uses D/C to 1GHz for signal processing, followed by up-conversion (U/C) for the down-link. Each sub-band will contain many messages, which will be fed together to the HPA (high power amplifier) for amplification.

omms.pdf Schematic of two satellite transponders. The top one is a C-Band system and the one on the bottom is a Ku-Band system. HPA = High power amplifier; DC = Downconverter; U/C = Upconverter.

In the C band 6/4GHz transponder (Figure 9 A): the uplink is at the higher frequency, so D/λ is greater for the (common) receive/transmit antenna – it will have a higher gain the input filter is a fairly wideband bandpass ‘ roofing’ filter to allow all the uplink frequencies in, but eliminating out-of-band noise LNA – low noise amplifier D/C – down converter to 4GHz (the down-link frequency) for signal processing – error correction, amplification, signal channelling etc. frequency demultiplexing – divides input signal into sub-bands to reduce non-linear distortion during amplification. Each sub-frequency band is processed by a single transponder. equalisers – correct for phase differences between the different frequency components of a signal which are introduced by filters, de-multiplexers etc HPAs – high power amplifiers – to increase power levels before re-transmission on the down-link. Non-linear performance in the HPAs can intoduce harmonics, intermodulation distortion etc

The Ku (14/11GHz) system (Figure 9 – B) has many of the same elements, but the down-link frequency (11GHz) is too high for the elements in each transponder,so the input is mixed down from 14GHz to 1GHz for de-multiplexing and equalisation, then mixed up to 11GHz for power amplification, frequency MUX and re-transmission. band-pass filters at various points remove out- ofband products from the HPAs etc and reduce the background noise, but they cannot remove in-band products – eg 3rd order intermodulation (IM) products