Hierarchical modulation: Difference between revisions

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{{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]].

Hierarchical modulation is also taken as one of the practical implementations of [[dirty paper coding | superposition precoding]], which can help achieve the maximum sum rate of broadcast channels. When hierarchical-modulated signals are transmitted, users with good reception and advanced receivers can demodulate multiple layers. For a user with a conventional receiver or poor reception, it may only demodulate the data stream embedded in the base layer. With hierarchical modulation, a network operator can target users of different types with different services or [[quality of service|QoS]].

However, traditional hierarchical modulation suffers from serious inter-layer interference (ILI) with impact on the achievable symbol rate.

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==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==

* [[Link adaptation]]

* [[Scalable Video Coding|H.264 Scalable Video Coding]]

* [[Mipmap]], [[Pyramid (image processing)]] – similar techniques in image processing

* [[High Efficiency Video Coding|H.265]] scalability extensions (SHVC)

* [[AV1#Scalable video coding|AV1 Scalable video coding]]

* [[MPEG-4 SLS]]

* [[LCEVC|MPEG-5 Part 2 / Low Complexity Enhancement Video Coding / LC EVC]]

* [[DTS-HD Master Audio|DTS-HD MA]]

* [[Bitrate peeling|Ogg Vorbis bitrate peeling]]

* [[WavPack#Hybrid_mode|WavPack hybrid mode]]

* [[JPEG 2000#Progressive transmission by pixel and resolution accuracy|JPEG 2000 SNR scalability]]

==References==

*H. Méric, J. Lacan, F. Arnal, G. Lesthievent, M.-L. Boucheret, [https://dx.doi.org/10.1109/TBC.2013.2268694 Combining Adaptive Coding and Modulation With Hierarchical Modulation in Satcom Systems], ''IEEE Transactions on Broadcasting'', Vol. 59, No. 4 (2013), pp 627-637.

* Shu Wang, Soonyil Kwon and Yi, B.K., [https://dx.doi.org/10.1109/ISBMSB.2008.4536685 On enhancing hierarchical modulation], ''IEEE International Symposium on Broadband Multimedia Systems and Broadcasting'', March 31 2008-April 2 2008, Las Vegas, NV, (2008), pp. 1-6.

*Seamus O Leary, '[https://ieeexplore.ieee.org/document/598365/metrics#metrics Hierarchical transmission and COFDM systems]', Published in: IEEE Transactions on Broadcasting ( Volume: 43, Issue: 2, Jun 1997)Page(s): 166 - 174

==External links==