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Hierarchical modulation
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<p>Hierarchical modulation, also called layered modulation, is one of the <a href="/facts/Signal_processing/Npaja6zb">signal processing</a> techniques for <a href="/facts/Multiplexing/YE7hK8cw">multiplexing</a> and <a href="/facts/Modulation/aAXPjINv">modulating</a> multiple data streams into one single symbol stream, where base-layer symbols and enhancement-layer symbols are synchronously overlaid before transmission. </p><p>Hierarchical modulation is particularly used to mitigate the <a href="/facts/Cliff_effect/Jo0XUcG9">cliff effect</a> in <a href="/facts/Digital_television/KQCyILQa">digital television</a> broadcast, particularly <a href="/facts/Mobile_TV/N06lgVtM">mobile TV</a>, by providing a (lower quality) fallback signal in case of weak signals, allowing <a href="/facts/Graceful_degradation/rasrYlJk">graceful degradation</a> instead of complete signal loss. It has been widely proven and included in various standards, such as <a href="/facts/DVB-T/oLaP9twt">DVB-T</a>, <a href="/facts/MediaFLO/6gmLkzhn">MediaFLO</a>, UMB (<a href="/facts/Ultra_Mobile_Broadband/doRtY7kt">Ultra Mobile Broadband</a>, a new 3.5th generation mobile network standard developed by 3GPP2), and is under study for <a href="/facts/DVB-H/uRGX039M">DVB-H</a>. </p><p>Hierarchical modulation is also taken as one of the practical implementations of <a href="/facts/Dirty_paper_coding/XuANaYso"> superposition precoding</a>, 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 <a href="/facts/Quality_of_service/joz9VkUm">QoS</a>. </p><p>However, traditional hierarchical modulation suffers from serious inter-layer interference (ILI) with impact on the achievable symbol rate. </p>