- Hierarchical modulation
Hierarchical modulation, also called layered modulation, is one of the
signal processing techniques formultiplexing and modulating multiple data streams into one single symbol stream, where base-layer symbols and enhancement-layer symbols are synchronously overplayed to together before transmission.Hierarchical modulation 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 forDVB-H .Hierarchical modulation is also taken as one of the practical implementation of 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 receiver can demodulate multiple layers. For a user with conventional receiver or poor reception condition, 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 QoS's.
However, traditional hierarchical modulation suffers from serious inter-layer interference (ILI) so that the achievable rate by low-layer data stream, e.g. the base-layer data stream, is dented by the interference from high-layer signal(s).
For example, for a hierarchically modulated symbol with 16QAM base layer and QPSK enhancement layer, the base-layer throughput loss is up to about 1.5bits/symbol with the total receive
signal-to-noise ratio (SNR) at about 23 dB, which is necessary for taking the full potentials of 64-ary QAM. 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.Example
For example, the figure depicts a layering scheme with QPSK base layer, and a 16QAM 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'.
ee also
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Link adaptation External links
* [http://www.broadcastpapers.com/whitepapers/paper_loader.cfm?pid=520 Hierarchical Modulation Explained]
* [http://www.dvb.org/technology/fact_sheets/WP01_DVB-T%20Hierarchical.pdf Hierarchical Modulation under DVB]
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