Hierarchical modulation

Lore & Background

Hierarchical modulation has been widely proven and included in various standards, such as DVB-T, MediaFLO, UMB (Ultra Mobile Broadband, a 3.5th generation mobile network standard developed by 3GPP2), and is under study for DVB-H. It is also taken as one of the practical implementations 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 receivers can demodulate multiple layers, while users with conventional receivers or poor reception 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.

Traditional hierarchical modulation suffers from serious inter-layer interference (ILI) with impact on the achievable symbol rate. For a hierarchically-modulated symbol with QPSK base layer and 16QAM enhancement layer, the base-layer throughput loss is up to about 1.5 bits/symbol with the total receive signal-to-noise ratio (SNR) at about 23 dB, about the minimum needed for the comparable non-hierarchical modulation, 64QAM. Unlayered 16QAM with the same SNR would approach full throughput, meaning due to ILI, about 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.

Reader's Guide

Hierarchical modulation's significance lies in its ability to provide graceful degradation in digital television broadcast, particularly for mobile TV, where signal strength varies. By overlaying a base layer and an enhancement layer, it allows receivers with poor signal conditions to still decode a lower-quality signal, avoiding the cliff effect of complete signal loss. This technique has been adopted in multiple standards including DVB-T, MediaFLO, and UMB, and was under study for DVB-H. It also serves as a practical implementation of superposition precoding, enabling broadcast channels to approach maximum sum rate. However, its legacy is tempered by the serious inter-layer interference (ILI) that reduces achievable symbol rate. For example, with a QPSK base layer and 16QAM enhancement layer, the base-layer throughput loss can reach about 37.5% at an SNR of 23 dB, compared to unlayered 16QAM. This ILI also increases demodulation error rates for higher-layer symbols. Despite these drawbacks, hierarchical modulation remains a proven method for targeting different user types with different services or quality of service levels within a single transmission.

Did You Know?

• Hierarchical modulation is also called layered modulation.
• The base layer in a QPSK/64QAM example carries 2 bits, while the enhancement layer carries 4 bits.