Variable Coding and Modulation


Variable Coding and Modulation

Variable Coding and Modulation is a type of data frame structure, where the size, coding and modulation type of the frame is best varied to the conditions.

While VCM is a one-way process, if a feedback loop (or return channel) is introduced then this can taken further to achieve Adaptive Coding and Modulation, so that the efficency of the data transmission can be maximised.

VCM and ACM in DVB-S2

Both VCM and ACM have been implemented by the new DVB-S2 (digital satellite television) standard;

Variable Coding and Modulation (VCM)

In DVB-S2 each frame that is transmitted, contains information on how that frame is modulated and coded in itsheader. A VCM capable demodulator will read this information from the header of each frame and use it todemodulate and decode that frame. This allows varying the modulation and coding parameters of eachtransmitted frame within the DVB-S2 stream since decoding is done on a frame-by-frame basis.A unique feature of the DVB-S2 standard is that multiple services can be transmitted on the same carrier, eachusing their own modulation scheme and coding rate. VCM is particularly powerful when different services do notneed the same protection level (for example, it is acceptable to lose a secondary channel in case of rain fade), or,like in our example, different services are intended for different stations with different average receivingconditions.


= Adaptive Coding and Modulation (ACM) =

When a return channel is available from each receiving site to the transmit site, DVB-S2 offers an even morepowerful feature known as Adaptive Coding and Modulation (ACM). With ACM it is possible to dynamically modifythe coding rate and modulation scheme for every single frame, according to the measured channel conditionswhere the frame is received. The return channel is used to dynamically report the receiving conditions at eachreceiving site.The DVB-S2 standard (ETSI EN302307) reads“In the case of interactive and point-to-point applications, the VCM functionality may be combined with the use of return channels, toachieve Adaptive Coding and Modulation (ACM). This technique provides more exact channel protection and dynamic link adaptation topropagation conditions, targeting each individual receiving terminal. ACM systems promise satellite capacity gains of up to 100 %-200 %. Inaddition, service availability may be extended compared to a constant protection system (CCM) such as DVB-S or DVB-DSNG. Such gainsare achieved by informing the satellite up-link station of the channel condition (e.g. C/N+I) of each receiving terminal via the satellite orterrestrial return channels.”

References

*Citation
last1 = Morello | first1 = A.
last2 = Mignone | first2 = V.
title = DVB-S2 - ready for lift off
publisher = EBU Technical Review
year = 2004


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