Hierarchical modulation: Difference between revisions

Revision as of 02:42, 23 September 2021

Latest revision as of 18:52, 24 June 2024

(6 intermediate revisions by 5 users not shown)

Line 1:

{{Short description|Signal processing technique}}

{{short description|Signal processing technique for multiplexing/modulating multiple data streams into one stream, where base- and enhancement-layer symbols are synchronously overplayed before transmission; used in digital TV broadcast for graceful degradation}}

{{UnreferencedNo citations |date=JuneFebruary 20082024}}

{{Modulation techniques}}

'''Hierarchical modulation''', also called '''layered modulation''', is one of the [[signal processing]] techniques for [[multiplexing]] and [[modulation|modulating]] multiple data streams into one single symbol stream, where base-layer symbols and enhancement-layer symbols are synchronously overplayedoverlaid before transmission.

Hierarchical modulation is particularly used to mitigate the [[cliff effect]] in [[digital television]] broadcast, particularly [[mobile TV]], by providing a (lower quality) fallback signal in case of weak signals, allowing [[graceful degradation]] instead of complete signal loss. It has been widely proven and included in various standards, such as [[DVB-T]], [[MediaFLO]], UMB ([[Ultra Mobile Broadband]], a new 3.5th generation mobile network standard developed by 3GPP2), and is under study for [[DVB-H]].

Line 12:

==Example==

[[Image:Layering.png|frame|right|Layered Modulationmodulation Constellation]]constellation: {{hlist

| list_style=display:inline; | style=display:inline;

| {{colorbull|#00ff00|circle}} 2 bits, QPSK

| {{colorbull|#e8eef7|}} 4 bits, 64QAM

}}]]

For example, the figure depicts a layering scheme with [[Phasephase-shift keying|QPSK]] base layer, and a [[Quadraturequadrature amplitude modulation|64QAM]] enhancement layer. The first layer is 2 bits (represented by the green circles). The signal detector only needs to establish which quadrant the signal is in, to recover the value (which is '10', the green circle in the lower right corner). In better signal conditions, the detector can establish the phase and amplitude more precisely, to recover four more bits of data ('1101'). Thus, the base layer carries '10', and the enhancement layer carries '1101'.

==Inter-layer interference==

{{expand section|date=September 2009}}

For a hierarchically-modulated symbol with QPSK base layer and 16QAM enhancement layer, the base-layer throughput loss is up to about 1.5bits5 bits/symbol with the total receive [[signal-to-noise ratio]] (SNR) at about 23 [[Decibeldecibel|dB]], about the minimum needed for the comparable non-hierarchical modulation, 64QAM. But unlayered 16QAM with the same SNR would approach full throughput. This means, due to ILI, about 1.5/4 = 37.5% loss of the base-layer achievable throughput. Furthermore, due to ILI and the imperfect demodulation of base-layer symbols, the demodulation error rate of higher-layer symbols increases too.

==See also==