Optimizing Enhanced Hierarchical Modulations

Though hierarchical modulation offers an important coverage and throughput tradeoff for wireless communications, it has received relatively little attention to date. Traditional hierarchical modulation suffers from the interference between layers, which results in both capacity loss and bit-error rate increasing. In this paper, an enhanced hierarchical modulation technique along with four optimization criteria are discussed. The proposed hierarchical modulation enhancement, where the enhancement-layer signal constellation is intentionally rotated, helps achieve better performance with little complexity increase. The first criterion is to maximize the achievable spectral efficiency with optimizing the Euclidean distance profile. The second criterion is to lower demodulation error rate with maximizing the modulation efficiency, a concept suggested for quantizing inter- layer interference. The third criterion is proposed to maximize the demodulation robustness when the imperfect channel estimation happens. The last approach is suggested for maximizing the RF power amplifier efficiency at the transmitter side with reducing the peak-to-average-power ratio of modulated signals when multi-carrier transmission is applied. All proposed schemes are simple and efficient. They can help recover the performance loss by regular hierarchical modulations with little complexity increase. Computer simulation is provided to support our conclusions.