Power Indexed Adaptive Modulation to Improve Spectral Efficiency

This letter proposes a novel transmission scheme, named power-indexed adaptive modulation (PIAM), to improve the spectral efficiency in fading channels, where the power index modulation (PIM) is integrated with the adaptive modulation (AM) and two data streams are simultaneously conveyed in a symbol transmission. The first data stream is mapped onto conventional constellations and the modulation order is adapted according to the instantaneous channel state. The second data stream is conveyed by the selection of transmit power within the interval between the minimum transmit power required by the error performance and the maximum transmit power of the system. In this way, the delivery of the second data stream does not affect that of the first one, while improving the spectral efficiency without any consumption of communication resources. Theoretical analysis and simulation results are provided to substantiate the effectiveness of our proposed scheme.