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The amplitude and phase shift keying (APSK) signal has been adopted in the recent satellite communication standards such as DVB-S2 due to its peak-to-average power ratio (PAPR) property lower than the quadrature amplitude modulation (QAM). Unlike square QAM constellations that allow separate detection of in-phase and quadrature components (i.e., I-Q decomposition), the detection process for APSK is generally complex. This paper investigates the use of multilevel coding (MLC) together with multistage decoding (MSD) for APSK with particular emphasis on an introduction of a novel labeling that allows I-Q decomposition at the highest level, thereby significantly reducing the decoding complexity at the cost of slight performance degradation.

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We address the use of a multilevel coding (MLC) to the amplitude phase-shift keying (APSK) modulations that consist of multiple PSK rings of different radii with a distinct number of constellation points. Such modulations have been adopted in the satellite broadcasting standards since they have a significant advantage over commonly used square-type quadrature amplitude modulations; their signals exhibit lower peak-to-average power ratio and thus enhance the power amplifier efficiency at the transmitter. A common low-complexity approach for decoding of MLC is the multi-stage decoding (MSD). However, MSD in general leads to severe latency at the decoder since the binary decisions on the higher levels should wait for those of the lower levels. Furthermore, this latency issue is even more salient for APSK signaling due to its circular nature of constellations that does not allow its decomposition into in-phase and quadrature components (i.e., I-Q decomposition). Therefore, in this paper we introduce a new MLC design for APSK that enables the decoder to reduce its latency without any increase in terms of complexity. The proposed approach consists of a novel labeling with an unconventional set partitioning that allows the detector to make use of I-Q decomposition. Specifically, the proposed set partitioning results in a quadrature-PSK constellation at the penultimate level, which can be decomposed into a pair of separately decodable binary-PSK constellations at the final level. The resulting decoder consists of a combination of MSD and parallel independent decoding that can be implemented with lower latency. The simulation results together with the theoretical analyses demonstrate that the reduction of not only the decoding latency but also the complexity in bit metric calculation can be achieved without noticeable degradation in terms of error performance compared to the conventional set partitioning approach.Index Terms-Amplitude phase-shift keying (APSK), digital video broadcasting-satellite-second generation (DVB-S2), in-phase and quadrature (I-Q) decomposition, multilevel coded modulation (MLC), turbo code.