21 September 2014

ISB (Independent Side Band) transmissions

As said in a previous post about Portoguese Navy [1] , the ISB (Independent Side Band transmission) system uses two SSB channels, which are independed to each other, added to form two side bands around the reduced carrier. So it can simultaneouly convey totally different transmissions.


Two different input signals modulate the same carrier frequency to produce two double-sideband (DSB) waveforms, then two band-pass filters are used to select the upper sideband and the lower sideband in the other.


The selected sidebands are combined to produce a final DSB signal in which each sideband carries different information. 
Since, the number of single-sideband (SSB) channels that can be transmitted over a given communication system is determined by the minimum frequency separation of the channels, using ISB mode, channels can be spaced closer together, and hence a further bandwidth economy can be achieved.

ISB mode is useb by Portuguese Navy in their STANAG-4285 trasmissions to carry encrypted traffic in the upper side band and CARBs messages [2] in the lower side band as in the following example:


Other examples of the use of the ISB and DSB (i.e. the same information on both the two lateral bands) mode can be found tuning some Link-11 [3] transmissions







[1] http://i56578-swl.blogspot.it/2014/08/portuguese-navy-cta-nato-on-hf-stanag.html
[2] http://i56578-swl.blogspot.it/search/label/CARB
[3] http://i56578-swl.blogspot.it/search/label/Link-11 

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