satellite internet : carriers in system design

Hi There,

First you have to decide the most excellent satellite covering the considered necessary location by means of the most EIRP, followed by performing a budgetary link budget which will depends on more than a few factors as:

•EIRP
•G/T
•SFD
•Other factors to be considered by the satellite operator himself
•Location of the transmitter
•Location of the receiver
•Type of modulation
•Type of the used modem

A satellite transponder is always used most efficiently if you operate with one single large carrier, e.g. 27.5 Msps.

Operation with multiple carriers, such as 4 or 8, requires that the composite transponder output power be backed off by about 3 dB. Multiple carriers intermodulate with one another and cause inteference into each other unless the transponder is backed off into the linear part of its transfer characteristic. This means that, when operated multicarrier, that the power available from the transponder is about half, compared with single carrier operation.  Exceptions: In the case of 2 carriers, the 2A-B intermodulation products fall either side of the wanted carriers and the effect is loss of power only, with little interference. Some satellites have on-board linearisers that allow the satellite HPA to be operated nearer to saturation. If you have a vast surplus of bandwidth and little power then there are some special carrier frequency patterns, called Babcock spacings, whereby multiple carriers may be arranged so that the intermodulation products fall between the carriers.  This works best for analogue FM. Typically 85% of the bandwidth is wasted and unusable due to intermodulation interference.

Back off in multicarrier mode applies to your hub HPA/BUC amplifer as well.  A hub may have to tranmit multiple carriers, to feed traffic via different transponders for example, and in such cases must operate its HPA/BUC well backed off.

So 4 or 8 carriers is no good for the outlink.  You should combine them altogether into one large 32 Mbps carrier.  The traffic content may be divided up (different services, different customers, different traffic type priorities, QoS etc) using a Cisco router.

It is obviously cheaper to have one transmit modem at the hub rather than 4 or 8.  A lower power BUC/HPA at the hub will also cost less, as will the mains electrical power and cooling costs.

Best regards, Eric.