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Broadcast multicast service (BCMCS) has increasingly been popular for delivering multimedia content to mobile users. Traditional digital broadcast air interfaces are designed with the tradeoff between maximum achievable rate and intended coverage in mind. The actual rates are usually limited by the maximum transmit power and the worst channel condition so that every user in coverage can reliably receive the services as well as contents of same quality. The users under good reception condition may have no advantage, even if their potential throughputs can be much higher. This happens often, especially on the mobile users whose reception conditions change all the time. And there are rising interests in upgrading existing digital broadcast systems with more services for new users and delivering more quality of service (QoS) options to users with advanced receivers while still guaranteeing existing users' services. Furthermore, recent advances in wideband speech coding, e.g., EVRC-WB, and scalable video coding, e.g., H.264/MPEG-4 AVC, suggest unequal error protection on content delieveries with providing graceful degradation of quality in the presence of increasing packet loss. It is possible for the users in good reception condition have more opportunities to enjoy high quality services while the user with low throughput can still decode the content of basic quality. Many technologies are under investigation for these goals, e.g., rateless coding, hierarchical modulation, multiple-input multiple-output (MIMO), selective retransmission and superposition precoding (SPC). Backward compatibility, implementation complexity and upgrading cost are among the major concerns in upgrading existing systems with additional services. Among those candidates, hierarchical modulation, also called layered modulation, is the most popular one, in which multiple data streams are multiplexed and modulated into one single symbol consisting of base-layer subsymbols and enhancement-layer subsymbols. 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), etc., and is under study for DVB-H.
Figure 1. Enhanced hierarchical modulation example: QPSK/QPSK
In this contribution, the regular hierarchical modulation is firstly extended by allowing additional rotation on the enhancement layer signal constellation. The generalized hierarchical modulations are then studied and analyzed from four different perspectives, such as achievable capacity, modulation efficiency, demodulation robustness and peak-to-average-power ration (PAPR) when it is combined with the popular orthogonal frequency-division modulation (OFDM) transmission scheme. At first, the achievable capacities of hierarchical modulations over Gaussian broadcast channel are studied from an information-theoretical perspective. As an example, the capacity of a regular 16QAM is tore down into the equivalent capacities of a base layer and enhancement layer. It is shown that there is a capacity loss on the base layer due to the inter-layer interference (ILI) from the enhancement layer. And this capacity loss can be mitigated by properly rotating the enhancement signal constellation. From a signal-processing perspective, it is known that the capacity loss is also related to the Euclidean distance profile of the hierarchical modulation signal constellation. For example, in high signal-to-noise ration (SNR) region, the symbol error rate usually is dominated by the minimum Euclidean distance. Obviously, with properly rotating the enhancement layer signal constellation and maximizing the minimum Euclidean distance, the resulted symbol error rate will decrease. Additionally, for tracking Euclidean distance profile changes, several parameters like effective signal power, effective SNR and modulation efficiency are discussed too. After this, hierarchical modulations are analyzed from an implementation perspective with considering channel estimation errors, which includes both channel amplitude estimation errors and channel phase estimation errors. It is shown that the demodulation robustness of hierarchical modulations can also be controlled by changing the Euclidean distance profile. Finally hierarchical modulations are discussed from a transmit power efficiency perspective when it is combined with multicarrier transmission. With avoiding high back-offs and maximizing average output power, it shows that high RF transmitter power efficiency is achievable by properly rotating the enhancement layer signals. With the analyses from different aspects of hierarchical modulation, a in-depth understanding of it can be achieved.
Figure 2. Capacity tear-down of 16QAM, a hierarchical modulation perspective