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Satellite Communications/Communication Satellite Performance Specifications


QUESTION: All the communication satellites has a performance specification of frequency response (dB/MHz) for its transponders. Is this specification needed for QPSK services as it is not effected by amplitude variation? If yes, then why? Even it is also specified for the filters after High Power Amplifier.

ANSWER: Hi Mr Rajesh,
I am not sure as to what exactly you may be referring to as "frequency response (dB/MHz) for its transponders". However the specifications for designing Link (or Satellite parameters for Link)  do not mention this parameter of frequency response but only about the Transponder usable bandwidth and power. Also there is a limitation specified in IBO and OBO. So the modulation scheme like QPSK are considered only for Link Budget estimates for margin calculation and BER. In specifications, Transponder usable BW is treated as unitary system and the frequency performance is considered linear throughout the Bandwidth.
However the specification for HPAs and filters concerns the roll of factor. This has implication on the guard-band between inter carrier/inter transponder space to avoid interference. Roll of factor defines the filter characteristics of the compatible Earth station so that there is no spill off to the neighborhood carriers/transponders that may indirectly interfere or saturate them. Similarly HPA is specified for OBO margin and the transponder usage in Multi carrier environment.

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Thanks for the reply. Actually, my question was regarding the transponder performance specification.
Frequency response specification signifies the bandpass flatness of the transponder i.e. amplitude ripples in a channel. Can this amplitude ripple cause degradation to the QPSK modulated signal?

ANSWER: Hi Mr Rajesh,
Straiaght answer to your question is "NO". Because the Transponder can be treated simply as "Converter- Amplifier", in the sense that Uplink is converted to downlink frequency and amplifies the low strength signal for transmission to Footprint area.

So the performance specification for Transponder are no different than for any amplifier with the respective bandwidth and spectrum paramaters. The mentioned parameter is actually a design specification "for the designer" to obtain the required response from the transponder, thats all. Transponders follow the same design principle irrespective of the modulation schemes, because the services that may be offered in a given Xponder may vary based on the services. However, the compatibility of certain transponders is fine tuned for linear/ non linear frequency performance of the user requirements ie., whether the BW is wideband application or UWB or NB so that linear response will not deteriorate the signal characteristics irrespective of the modulation. QPSK is digiatl modulation technique but follows the same analog principle characteristics in terms of Physical Layer Services.

The specification you mention are purely design spec and has no bearing on the modulation schema. The characteristic of QPSK is of no significane to the Link.

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Thanks for a nice reply. But does it means that amplitude ripple in transponder will not degrade the BER performance of the QPSK modulated signal. As if we simulate a system with QPSK modulated signal passing through a system with amplitude ripple, its Eb/No is degraded for a particular BER. Is it mean that my simulation is having some error?

Hi Mr Rajesh,
The transponder designers' responsibilty is to take of ripple factor and flatness parameters with respect to the frequency of operations. In multi carrier environment, Transponder is taken as single UNIT and not as ezch carrier and its modulation characteristics. So the designer's specs for transponder are valid for any type of application. I am not sure what you may be referring to about Simulator please, is it QPSK signal simulator or Trnsponder simulator?

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Satellite Communications Applications, Link Calculations, Teleports, Earth Stations,VSAT, Tracking, Mobile Communications, Next Generation Networks.


40 Years. Indian Space Research Organisation, PANAMSAT, Satellite Based VSAT Networks, IP Networks.

ISRO, PANAMSAT, Engineering Institution for Higher Education (GVP College of Engineering), Consultancy.

Graduate in Electronics and Communication Engineering with Distinction. Guiding scholars for their Doctoral thesis in related areas in Academics.

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